diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index c0d8c9e..d2a3c4c 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -553,7 +553,6 @@ function template_preprocess_block(&$variables) {
 
   $variables['attributes'] = new Attribute;
   $variables['attributes']['class'] = array();
-  $variables['attributes']['class'][] = 'block';
   $variables['attributes']['class'][] = drupal_html_class('block-' . $variables['block']->module);
 
   $variables['theme_hook_suggestions'][] = 'block__' . $variables['block']->region;
diff --git a/core/modules/block/templates/block.html.twig b/core/modules/block/templates/block.html.twig
index 01eb3c9..b7b2342 100644
--- a/core/modules/block/templates/block.html.twig
+++ b/core/modules/block/templates/block.html.twig
@@ -37,7 +37,7 @@
  @todo Remove the div around content - and make sure this doesn't break any
   CSS defined in modules.
 #}
-<div id="{{ attributes.id }}" class="{{ attributes.class }}"{{ attributes }}>
+<div id="{{ attributes.id }}" class="block {{- attributes.class }}"{{ attributes }}>
   {{ title_prefix }}
   {% if label %}
     <h2{{ title_attributes }}>{{ label }}</h2>