/**
 * Implements hook_block_alter().
 */
function {{ machine_name }}_block_alter(&$definitions) {
  foreach ($definitions as $id => $definition) {
    if (strpos($id, 'system_menu_block:') === 0) {
      // Replace $definition properties: id, deriver, class, provider to ones
      // provided by this custom module.
    }
  }
}