Add class all block in specified region

gistfile1.php

/**
 * Implements hook_preprocess_block().
 *
 * @see block.tpl.php
 */
function base_theme_preprocess_block(&$variables) {
  if ($variables['elements']['#block']->region == 'footer') {
    $variables['classes_array'][] = 'col-xs-12 col-sm-6 col-md-3';
  }
}