gistfile1.php

<?php

function block_menu() {
  $items['block/autocomplete'] = array(
    'title' => 'Block autocomplete',
    'page callback' => 'block_autocomplete',
    'access callback' => 'user_access', 
    'access arguments' => array('administer blocks'),
    'type' => MENU_CALLBACK, 
  );
  return $items;
}

function block_autocomplete($string = '') {
  $matches = array();
  if ($string) {
    $blocks = block_manager()->getDefinitions();
    $block_titles = array();
    foreach($blocks as $plugin_id => $block) {
      $block_titles[$plugin_id] = $block['subject'];
    }
    $matches = preg_grep("/^". $string . "/i", $block_titles);
  }
  return new JsonResponse($matches);
}