drupal 7: adding classes to blocks with preprocessing. borrowed from http://www.webbykat.com/2012/09/adding-class-block-drupal-7-block-class-module-vs-preprocessing

preprocess-block.inc

<?php
/**  
 *  Selectively add classes to certain blocks
 *
 *  Implements alpha_preprocess_block
 */
function sil_alpha_preprocess_block(&$var) {
  $block_id = $var['block']->module . '-' . $var['block']->delta;
  $classes = &$var['attributes_array']['class'];
  switch ($block_id) {
    /* Add .badge class to block #14 */
    case 'block-14':
      $classes[] = 'badge';
      break;
    /* Add .form class to block #20 */
    case 'block-20':
      $classes[] = 'form';
      break;
    default:
      // Print out block id's
      // var_dump($block_id);
      break;
  }
}