Single Bulk Operation on several blocks
If you have a specific list of blocks you want to alter, get their deltas and invoke hook_block_view_alter in a custom module, as follows:

function YOURMODULE_block_view_alter(&amp;$data, $block) {
  switch($block-&gt;delta) {
     case 'delta1':
     ...
     case 'deltaN':
       // alter the block in the required way by modifying the $data array
       break;
  }
 }


If you want to alter ALL blocks created by a particular module, say, views, do a switch on $block-&gt;module, instead, i.e.:

function YOURMODULE_block_view_alter(&amp;$data, $block) {
      switch($block-&gt;module) {
         case 'views':
         case 'SomeOtherModule':
           // alter the block in the required way by modifying the $data array
           break;
      }
}


Bear in mind that hook_block_view_alter is called for ALL blocks, so if your site has lots of blocks, this may be a performance hit.