diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index a08254a..3cc1482 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -89,6 +89,10 @@ function block_theme() {
       'render element' => 'elements',
       'template' => 'block',
     ),
+    'block_list' => array(
+      'render element' => 'form',
+      'template' => 'block-list',
+    ),
   );
 }
 
diff --git a/core/modules/block/css/block.admin.css b/core/modules/block/css/block.admin.css
index 7cd9233..fe1ef3e 100644
--- a/core/modules/block/css/block.admin.css
+++ b/core/modules/block/css/block.admin.css
@@ -32,12 +32,12 @@ a.block-demo-backlink:hover {
   text-decoration: underline;
 }
 
-.block-list-region {
+.layout-region {
   -webkit-box-sizing: border-box;
   -moz-box-sizing: border-box;
   box-sizing: border-box;
 }
-.block-list-right {
+.block-list-secondary {
   border: 1px solid #bfbfbf;
   border-bottom-width: 0;
 }
@@ -49,10 +49,7 @@ a.block-demo-backlink:hover {
   list-style: none;
   padding: 0.1em 0;
 }
-.block-list a:before {
-  content: '+ ';
-}
-.block-list-right .form-type-search {
+.block-list-secondary .form-type-search {
   padding: 0 1em;
 }
 
@@ -61,35 +58,35 @@ a.block-demo-backlink:hover {
 screen and (min-width: 780px),
 (orientation: landscape) and (min-device-height: 780px) {
 
-  .block-list-left {
-    float: left; /* LTR */
+  .block-list-primary {
+    float: right; /* LTR */
     width: 75%;
-    padding-right: 2em;
-  }
-  [dir="rtl"] .block-list-left {
-    float: right;
     padding-left: 2em;
-    padding-right: 0;
+  }
+  [dir="rtl"] .block-list-primary {
+    float: left;
+    padding-left: 0;
+    padding-right: 2em;
   }
 
-  .block-list-right {
-    float: right; /* LTR */
+  .block-list-secondary {
+    float: left; /* LTR */
     width: 25%;
   }
-  [dir="rtl"] .block-list-right {
-    float: left;
+  [dir="rtl"] .block-list-secondary {
+    float: right;
   }
 
   /* @todo File an issue to add a standard class to all text-like inputs */
-  .block-list-right .form-autocomplete,
-  .block-list-right .form-text,
-  .block-list-right .form-tel,
-  .block-list-right .form-email,
-  .block-list-right .form-url,
-  .block-list-right .form-search,
-  .block-list-right .form-number,
-  .block-list-right .form-color,
-  .block-list-right textarea {
+  .block-list-secondary .form-autocomplete,
+  .block-list-secondary .form-text,
+  .block-list-secondary .form-tel,
+  .block-list-secondary .form-email,
+  .block-list-secondary .form-url,
+  .block-list-secondary .form-search,
+  .block-list-secondary .form-number,
+  .block-list-secondary .form-color,
+  .block-list-secondary textarea {
     -webkit-box-sizing: border-box;
     -moz-box-sizing:    border-box;
     box-sizing:         border-box;
@@ -108,8 +105,8 @@ screen and (min-width: 780px),
 screen and (max-width: 1020px),
 (orientation: landscape) and (max-device-height: 1020px) {
 
-  .toolbar-vertical .block-list-left,
-  .toolbar-vertical .block-list-right {
+  .toolbar-vertical .block-list-primary,
+  .toolbar-vertical .block-list-secondary {
     float: none;
     width: auto;
     padding-right: 0;
diff --git a/core/modules/block/lib/Drupal/block/BlockListController.php b/core/modules/block/lib/Drupal/block/BlockListController.php
index b9d6218..9d2ed6a 100644
--- a/core/modules/block/lib/Drupal/block/BlockListController.php
+++ b/core/modules/block/lib/Drupal/block/BlockListController.php
@@ -120,6 +120,7 @@ public function getFormID() {
    */
   public function buildForm(array $form, array &$form_state) {
     $entities = $this->load();
+    $form['#theme'] = array('block_list');
     $form['#attached']['library'][] = array('system', 'drupal.tableheader');
     $form['#attached']['library'][] = array('block', 'drupal.block');
     $form['#attached']['library'][] = array('block', 'drupal.block.admin');
@@ -127,13 +128,7 @@ public function buildForm(array $form, array &$form_state) {
 
     // Add a last region for disabled blocks.
     $block_regions_with_disabled = $this->regions + array(BLOCK_REGION_NONE => BLOCK_REGION_NONE);
-    $form['left']['#type'] = 'container';
-    $form['left']['#attributes']['class'] = array(
-      'block-list-region',
-      'block-list-left',
-    );
-
-    $form['left']['block_regions'] = array(
+    $form['block_regions'] = array(
       '#type' => 'value',
       '#value' => $block_regions_with_disabled,
     );
@@ -144,11 +139,11 @@ public function buildForm(array $form, array &$form_state) {
     $weight_delta = round(count($entities) / 2);
 
     // Build the form tree.
-    $form['left']['edited_theme'] = array(
+    $form['edited_theme'] = array(
       '#type' => 'value',
       '#value' => $this->theme,
     );
-    $form['left']['blocks'] = array(
+    $form['blocks'] = array(
       '#type' => 'table',
       '#header' => array(
         t('Block'),
@@ -174,7 +169,7 @@ public function buildForm(array $form, array &$form_state) {
 
     // Loop over each region and build blocks.
     foreach ($block_regions_with_disabled as $region => $title) {
-      $form['left']['blocks']['#tabledrag'][] = array(
+      $form['blocks']['#tabledrag'][] = array(
         'match',
         'sibling',
         'block-region-select',
@@ -182,27 +177,27 @@ public function buildForm(array $form, array &$form_state) {
         NULL,
         FALSE,
       );
-      $form['left']['blocks']['#tabledrag'][] = array(
+      $form['blocks']['#tabledrag'][] = array(
         'order',
         'sibling',
         'block-weight',
         'block-weight-' . $region,
       );
 
-      $form['left']['blocks'][$region] = array(
+      $form['blocks'][$region] = array(
         '#attributes' => array(
           'class' => array('region-title', 'region-title-' . $region, 'odd'),
           'no_striping' => TRUE,
         ),
       );
-      $form['left']['blocks'][$region]['title'] = array(
+      $form['blocks'][$region]['title'] = array(
         '#markup' => $region != BLOCK_REGION_NONE ? $title : t('Disabled'),
         '#wrapper_attributes' => array(
           'colspan' => 5,
         ),
       );
 
-      $form['left']['blocks'][$region . '-message'] = array(
+      $form['blocks'][$region . '-message'] = array(
         '#attributes' => array(
           'class' => array(
             'region-message',
@@ -211,7 +206,7 @@ public function buildForm(array $form, array &$form_state) {
           ),
         ),
       );
-      $form['left']['blocks'][$region . '-message']['message'] = array(
+      $form['blocks'][$region . '-message']['message'] = array(
         '#markup' => '<em>' . t('No blocks in this region') . '</em>',
         '#wrapper_attributes' => array(
           'colspan' => 5,
@@ -222,19 +217,19 @@ public function buildForm(array $form, array &$form_state) {
         foreach ($blocks[$region] as $info) {
           $entity_id = $info['entity_id'];
 
-          $form['left']['blocks'][$entity_id] = array(
+          $form['blocks'][$entity_id] = array(
             '#attributes' => array(
               'class' => array('draggable'),
             ),
           );
 
-          $form['left']['blocks'][$entity_id]['info'] = array(
+          $form['blocks'][$entity_id]['info'] = array(
             '#markup' => check_plain($info['admin_label']),
             '#wrapper_attributes' => array(
               'class' => array('block'),
             ),
           );
-          $form['left']['blocks'][$entity_id]['region-theme']['region'] = array(
+          $form['blocks'][$entity_id]['region-theme']['region'] = array(
             '#type' => 'select',
             '#default_value' => $region,
             '#empty_value' => BLOCK_REGION_NONE,
@@ -246,12 +241,12 @@ public function buildForm(array $form, array &$form_state) {
             ),
             '#parents' => array('blocks', $entity_id, 'region'),
           );
-          $form['left']['blocks'][$entity_id]['region-theme']['theme'] = array(
+          $form['blocks'][$entity_id]['region-theme']['theme'] = array(
             '#type' => 'hidden',
             '#value' => $this->theme,
             '#parents' => array('blocks', $entity_id, 'theme'),
           );
-          $form['left']['blocks'][$entity_id]['weight'] = array(
+          $form['blocks'][$entity_id]['weight'] = array(
             '#type' => 'weight',
             '#default_value' => $info['weight'],
             '#delta' => $weight_delta,
@@ -261,33 +256,27 @@ public function buildForm(array $form, array &$form_state) {
               'class' => array('block-weight', 'block-weight-' . $region),
             ),
           );
-          $form['left']['blocks'][$entity_id]['operations'] = $this->buildOperations($info['entity']);
+          $form['blocks'][$entity_id]['operations'] = $this->buildOperations($info['entity']);
         }
       }
     }
 
     // Do not allow disabling the main system content block when it is present.
-    if (isset($form['left']['blocks']['system_main']['region'])) {
-      $form['left']['blocks']['system_main']['region']['#required'] = TRUE;
+    if (isset($form['blocks']['system_main']['region'])) {
+      $form['blocks']['system_main']['region']['#required'] = TRUE;
     }
 
-    $form['left']['actions'] = array(
+    $form['actions'] = array(
       '#tree' => FALSE,
       '#type' => 'actions',
     );
-    $form['left']['actions']['submit'] = array(
+    $form['actions']['submit'] = array(
       '#type' => 'submit',
       '#value' => t('Save blocks'),
       '#button_type' => 'primary',
     );
 
-    $form['right']['#type'] = 'container';
-    $form['right']['#attributes']['class'] = array(
-      'block-list-region',
-      'block-list-right',
-    );
-
-    $form['right']['title'] = array(
+    $form['place_blocks']['title'] = array(
       '#type' => 'container',
       '#children' => '<h3>' . t('Place blocks') . '</h3>',
       '#attributes' => array(
@@ -297,7 +286,7 @@ public function buildForm(array $form, array &$form_state) {
       ),
     );
 
-    $form['right']['filter'] = array(
+    $form['place_blocks']['filter'] = array(
       '#type' => 'search',
       '#title' => t('Filter'),
       '#title_display' => 'invisible',
@@ -310,8 +299,8 @@ public function buildForm(array $form, array &$form_state) {
       ),
     );
 
-    $form['right']['list']['#type'] = 'container';
-    $form['right']['list']['#attributes']['class'][] = 'entity-meta';
+    $form['place_blocks']['list']['#type'] = 'container';
+    $form['place_blocks']['list']['#attributes']['class'][] = 'entity-meta';
 
     // Sort the plugins first by category, then by label.
     $plugins = $this->blockManager->getDefinitions();
@@ -323,10 +312,11 @@ public function buildForm(array $form, array &$form_state) {
     });
     foreach ($plugins as $plugin_id => $plugin_definition) {
       $category = $plugin_definition['category'];
-      if (!isset($form['right']['list'][$category])) {
-        $form['right']['list'][$category] = array(
+      if (!isset($form['place_blocks']['list'][$category])) {
+        $form['place_blocks']['list'][$category] = array(
           '#type' => 'details',
           '#title' => $category,
+          '#collapsed' => TRUE,
           'content' => array(
             '#theme' => 'links',
             '#links' => array(),
@@ -338,7 +328,7 @@ public function buildForm(array $form, array &$form_state) {
           ),
         );
       }
-      $form['right']['list'][$category]['content']['#links'][$plugin_id] = array(
+      $form['place_blocks']['list'][$category]['content']['#links'][$plugin_id] = array(
         'title' => $plugin_definition['admin_label'],
         'href' => 'admin/structure/block/add/' . $plugin_id . '/' . $this->theme,
         'attributes' => array(
diff --git a/core/modules/block/templates/block-list.html.twig b/core/modules/block/templates/block-list.html.twig
new file mode 100644
index 0000000..873d192
--- /dev/null
+++ b/core/modules/block/templates/block-list.html.twig
@@ -0,0 +1,24 @@
+{#
+/**
+ * @file
+ * Two column template for the block add/edit form.
+ *
+ * This template will be used when a block edit form specifies 'block_edit_form'
+ * as its #theme callback.  Otherwise, by default, block add/edit forms will be
+ * themed by theme_form().
+ *
+ * Available variables:
+ * - form: The block add/edit form.
+ *
+ * @ingroup themeable
+ */
+#}
+{% hide(form.place_blocks) %}
+<div class="layout-block-list clearfix">
+  <div class="layout-region block-list-primary">
+    {{ form }}
+  </div>
+  <div class="layout-region block-list-secondary">
+    {{ form.place_blocks }}
+  </div>
+</div>
diff --git a/core/themes/seven/style.css b/core/themes/seven/style.css
index b0da782..c628c55 100644
--- a/core/themes/seven/style.css
+++ b/core/themes/seven/style.css
@@ -1763,7 +1763,7 @@ details.fieldset-no-legend {
   screen and (max-width: 1020px),
   (orientation: landscape) and (max-device-height: 1020px) {
 
-  .toolbar-vertical .block-list-right,
+  .toolbar-vertical .block-list-secondary,
   .toolbar-vertical .layout-region-node-secondary {
     margin-bottom: 0;
     padding-bottom: 0;