Yes, you can do that with preprocess_block() function.

just define title prefix and suffix.

**For example:**

    /**
     * Override or insert variables into the block template.
     */
    function YOURTHEME_preprocess_block(&$variables) {
    
      // all block titles in the content region will be wrapped in envelope icon
      if ($variables['block']->region == 'content') {  
        $variables['title_prefix'] = '<i class="icon-envelope">';
        $variables['title_suffix'] = '</i>';
      }
    }


also you can do that in [block template file][1] using for example [block--region.tpl.php][2]


  [1]: http://api.drupal.org/api/drupal/modules!block!block.tpl.php/7
  [2]: http://drupal.org/node/1089656