diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index b577124..94c0304 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -514,7 +514,8 @@ function template_preprocess_block(&$variables) {
 
   $variables['attributes']['class'][] = 'block';
   $variables['attributes']['class'][] = drupal_html_class($variables['configuration']['module']);
-  $variables['attributes']['class'][] = drupal_html_class($variables['plugin_id']);
+  $plugin_id_class = str_replace(':', '--', $variables['plugin_id']);
+  $variables['attributes']['class'][] = drupal_html_class($plugin_id_class);
   $variables['attributes']['id'] = drupal_html_id('block-' . $variables['elements']['#block']->id());
 
   // The block template provides a wrapping element for the content. Render the
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockHtmlIdTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockHtmlIdTest.php
index ab185a9..15f98f2 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockHtmlIdTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockHtmlIdTest.php
@@ -49,7 +49,7 @@ function setUp() {
   function testHtmlId() {
     $this->drupalGet('');
     $this->assertRaw('id="block-test-id-block"', 'HTML ID for test block is valid.');
-    $elements = $this->xpath('//div[contains(@class, :div-class)]/div/ul[contains(@class, :ul-class)]/li', array(':div-class' => 'system-menu-blocktools', ':ul-class' => 'menu'));
+    $elements = $this->xpath('//div[contains(@class, :div-class)]/div/ul[contains(@class, :ul-class)]/li', array(':div-class' => 'system-menu-block--tools', ':ul-class' => 'menu'));
     $this->assertTrue(!empty($elements), 'The proper block markup was found.');
   }