Does not work with OMEGA framework (and others)

This does not work out-of-the-box when using the OMEGA framework.

Basically, the custom block class doesn't get rendered in the HTML output.

For those with this issue, here's a simple solution that works:
https://www.drupal.org/node/1571498#comment-8168689

Meanwhile I request that this should be added to module documentation / page, as the Omega framework is v popular.