diff -u b/core/modules/block/block.module b/core/modules/block/block.module
--- b/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -273,9 +273,6 @@
     $variables['label'] = $variables['elements']['content']['#title'];
   }
 
-  // Add default class for block content.
-  $variables['content_attributes']['class'][] = 'content';
-
   // Create a valid HTML ID and make sure it is unique.
   if (!empty($variables['elements']['#id'])) {
     $variables['attributes']['id'] = drupal_html_id('block-' . $variables['elements']['#id']);
diff -u b/core/modules/block/templates/block.html.twig b/core/modules/block/templates/block.html.twig
--- b/core/modules/block/templates/block.html.twig
+++ b/core/modules/block/templates/block.html.twig
@@ -47,7 +47,7 @@
   {% endif %}
   {{ title_suffix }}
 
-  <div{{ content_attributes }}>
+  <div{{ content_attributes.addClass('content') }}>
     {% block content %}
       {{ content }}
     {% endblock %}