diff --git a/core/includes/common.inc b/core/includes/common.inc
index 8f60fe4..d821a42 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -1881,7 +1881,7 @@ function drupal_delete_file_if_stale($uri) {
  * @return
  *   The cleaned identifier.
  */
-function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-', '_' => '-', '__' => '__', '/' => '-', '[' => '-', ']' => '')) {
+function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-', '_' => '-', '__' => '__', '/' => '-', '[' => '-', ']' => '', ':' => '--')) {
   // By default, we filter using Drupal's coding standards.
   $identifier = strtr($identifier, $filter);
 
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 94c0304..b577124 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -514,8 +514,7 @@ function template_preprocess_block(&$variables) {
 
   $variables['attributes']['class'][] = 'block';
   $variables['attributes']['class'][] = drupal_html_class($variables['configuration']['module']);
-  $plugin_id_class = str_replace(':', '--', $variables['plugin_id']);
-  $variables['attributes']['class'][] = drupal_html_class($plugin_id_class);
+  $variables['attributes']['class'][] = drupal_html_class($variables['plugin_id']);
   $variables['attributes']['id'] = drupal_html_id('block-' . $variables['elements']['#block']->id());
 
   // The block template provides a wrapping element for the content. Render the