diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
index b5df464..95cab30 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
@@ -106,6 +106,30 @@ function testBlockAdminUiPage() {
   }
 
   /**
+   * Tests the block categories on the listing page.
+   */
+  public function testCandidateBlockList() {
+    $arguments = array(
+      ':ul_class' => 'block-list',
+      ':li_class' => 'test-block-instantiation',
+      ':href' => 'admin/structure/block/add/test_block_instantiation/stark',
+      ':text' => 'Display message',
+    );
+
+    $this->drupalGet('admin/structure/block');
+    $elements = $this->xpath('//details[@id="edit-block-test"]//ul[contains(@class, :ul_class)]/li[contains(@class, :li_class)]/a[contains(@href, :href) and text()=:text]', $arguments);
+    $this->assertTrue(!empty($elements), 'The test block appears in the category for its module.');
+
+    // Trigger the custom category addition in block_test_block_alter().
+    $this->container->get('state')->set('block_test_info_alter', TRUE);
+    $this->container->get('plugin.manager.block')->clearCachedDefinitions();
+
+    $this->drupalGet('admin/structure/block');
+    $elements = $this->xpath('//details[@id="edit-custom-category"]//ul[contains(@class, :ul_class)]/li[contains(@class, :li_class)]/a[contains(@href, :href) and text()=:text]', $arguments);
+    $this->assertTrue(!empty($elements), 'The test block appears in a custom category controlled by block_test_block_alter().');
+  }
+
+  /**
    * Tests that the BlockFormController populates machine name correctly.
    */
   public function testMachineNameSuggestion() {
diff --git a/core/modules/block/tests/modules/block_test/block_test.module b/core/modules/block/tests/modules/block_test/block_test.module
index 6abb95f..5184454 100644
--- a/core/modules/block/tests/modules/block_test/block_test.module
+++ b/core/modules/block/tests/modules/block_test/block_test.module
@@ -12,3 +12,12 @@ function block_test_system_theme_info() {
   $themes['block_test_theme'] = drupal_get_path('module', 'block_test') . '/themes/block_test_theme/block_test_theme.info.yml';
   return $themes;
 }
+
+/**
+ * Implements hook_block_alter().
+ */
+function block_test_block_alter(&$block_info) {
+  if (Drupal::state()->get('block_test_info_alter') && isset($block_info['test_block_instantiation'])) {
+    $block_info['test_block_instantiation']['category'] = t('Custom category');
+  }
+}