diff --git modules/block/block.module modules/block/block.module
index d2287f4..9c55909 100644
--- modules/block/block.module
+++ modules/block/block.module
@@ -745,10 +745,26 @@ function _block_get_cache_id($block) {
 function block_flush_caches() {
   return array('cache_block');
 }
-
 /**
  * Process variables for block.tpl.php
  *
+ * Prepare the final values passed to the theme_block function to be
+ * passed into a pluggable template engine.
+ *
+ * The $variables array contains the following arguments:
+ * - $block
+ * - $content
+ *
+ * @see block.tpl.php
+ */
+function template_process_block(&$variables) {
+  unset($variables['#attributes']['class']);
+  $variables['extra_attributes'] = empty($variables['#attributes']) ? '' : ' ' . drupal_attributes($variables['#attributes']);
+}
+
+/**
+ * Pre-process variables for block.tpl.php
+ *
  * Prepare the values passed to the theme_block function to be passed
  * into a pluggable template engine. Uses block properties to generate a
  * series of template file suggestions. If none are found, the default
@@ -778,7 +794,9 @@ function template_preprocess_block(&$variables) {
   $variables['content'] = $variables['elements']['#children'];
 
   $variables['classes_array'][] = 'block-' . $variables['block']->module;
-
+  if (isset($variables['#attributes']) && is_array($variables['#attributes']['class'])) {
+    $variables['classes_array'] = array_merge($variables['#attributes']['class'], $variables['classes_array']);
+  }
   $variables['template_files'][] = 'block-' . $variables['block']->region;
   $variables['template_files'][] = 'block-' . $variables['block']->module;
   $variables['template_files'][] = 'block-' . $variables['block']->module . '-' . $variables['block']->delta;
diff --git modules/block/block.tpl.php modules/block/block.tpl.php
index 74c8811..e2fb9ca 100644
--- modules/block/block.tpl.php
+++ modules/block/block.tpl.php
@@ -35,7 +35,7 @@
  * @see template_process()
  */
 ?>
-<div id="block-<?php print $block->module . '-' . $block->delta; ?>" class="<?php print $classes; ?>">
+<div id="block-<?php print $block->module . '-' . $block->delta; ?>" class="<?php print $classes; ?>"<?php print $extra_attributes; ?>>
 <?php if ($block->subject): ?>
   <h2><?php print $block->subject ?></h2>
 <?php endif;?>