overriding the block template

With your current implementation of hook_theme(), it is not possible to override the block template by copying it to the theme directory. I suggest the following changes to be considered (see this comment titled "naming your template files)

function core_release_block_theme() {
  $items['core_rel_block'] = array(
      'template' => 'core-rel-block',
  );
  return $items;
}

and renaming core-release-block.tpl.php as core-rel-block.tpl.php.