having multiple instances of blocks with different settings in D7 I have activated Multiblock module, create an instance of one custom block.
The admin/structure/block/instances say my new block instance is not 'multiblock enabled'

The doc say 
https://www.drupal.org/node/353565


  To get around this, you can make a block "multiblock enabled". To do
  this, you have to first add an $op to your hook_block called
  'mb_enabled'. It should always return the string 'mb_enabled'.


I don't quite get how to do that and I have tried that with no effect

/**
 * Implementation of hook_block().
 * @param string $op one of "list", "view", "save" and "configure"
 * @param integer $delta code to identify the block
 * @param array $edit only for "save" operation
 */
function amu_hal_block($op = 'list', $delta = 0, $edit = array()) {

    // set up an empty array which will contain the block contents
    $block = array();

    switch ($op) {
        case "list":
            // Generate listing of blocks from this module, for the admin/block page
            $block[0]["info"] = t('On This Date');
            break;

        case "view":
            break;

        case "save":
            break;

        case "configure":
            break;

            case 'mb_enabled':
                return 'mb_enabled';
                break;
    }

    return $block;
}


Is hook_block present in D7 at all?
There's no hook_block in Drupal 7. However if you take a look at your README file inside your D7 version of multiblock module, you can find how to do it:


  Example implementation of hook_block_info:

function hook_block_info() {
  $blocks['powered-by'] = array(
    'info' =&gt; t('Powered by Drupal'),
    'weight' =&gt; '10',
    'cache' =&gt; DRUPAL_NO_CACHE,
    'mb_enabled' =&gt; TRUE,
  );
  return $blocks;
}