Aggregator custom template file is not overriding the default one on Drupal 7
I don't think it uses template files.

Just override the function in your template.php.
(replace 'mytheme' with your theme's name).

/**
 * Returns HTML for an individual feed item for display in the block.
 *
 * @param $variables
 *   An associative array containing:
 *   - item: The item to be displayed.
 *   - feed: Not used.
 *
 * @ingroup themeable
 */
function mytheme_aggregator_block_item($variables) {
  // Display the external link to the item.
  return '&lt;a href="' . check_url($variables['item']-&gt;link) . '"&gt;' . check_plain($variables['item']-&gt;title) . "&lt;/a&gt;\n";
}