Provided by System module since v1.0

The Block Class module for Drupal is no longer needed in Backdrop CMS.

The ability to add custom classes to blocks has been supported in Backdrop since the initial release. It is not necessary to add any custom code to your theme.

Upgrade Instructions: 

There is no upgrade path for the Block Class module yet. Please follow this issue for progress on the upgrade path. Until the upgrade path is complete...

If you are running the Block Class module on a Drupal 7 site:

  1. Note all the custom classes you have added to your blocks in Drupal 7.
  2. Disable and Uninstall the Block Class module from Drupal 7.
  3. Upgrade your site to Backdrop CMS.
  4. Manually re-enter custom classes onto the blocks noted in step 1, using the fields provided by Backdrop core.