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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 using for example block--region.tpl.php