Default Menu Blocks

hook_menu_block_blocks

<?php
/**
 * Implements hook_menu_block_blocks().
 */
function sidebar_menu_menu_block_blocks() {
  // The array key is the block delta used by menu block.
  return array(
    'sidebar_subnav' => array(
      'title_link'  => TRUE,
      'admin_title' => 'Current Menu Children',
      'menu_name'   => MENU_TREE__CURRENT_PAGE_MENU,
      'level'       => 2,
      'follow'      => 0,
      'depth'       => 0,
      'expanded'    => TRUE,
      'sort'        => FALSE,
      'parent_mlid' => 0,
    ),
  );
}