Use mytheme_preprocess_block() to display HTML (icon fonts) in block titles
In preprocess_block() function you can only define the title prefix and suffix for each title.

For example:

/**
 * Override or insert variables into the block template.
 */
function YOURTHEME_preprocess_block(&amp;$variables) {

  $variables['title_prefix'] = array(
    '#type' =&gt; 'markup',
    '#markup' =&gt; '&lt;i class="icon-envelope"&gt;',
  );
  $variables['title_suffix'] = array(
    '#type' =&gt; 'markup',
    '#markup' =&gt; '&lt;/i&gt;',
  );

}