Is any depth change allowed on electrical panel wall?
The NEC's clearance requirements are for the safety of people working on equipment once it is installed. 

With that being said the NEC width requirement is a minimum of 30", and the door swing if there is one minimum is 90 degrees. However your panel does not have to sit in the center of the 30" it can be moved from side to side. 

Reading you question I can't determine whether you panel is against the block wall and the wall depth change is 6" and runs the other direction at that same depth at least 30" or there is just a recess that the panel fits in and it is recessed in an area less than 30". So with the first situation the panel is fine and with the latter, it needs to be brought out to meet clearance.

Also I don't know if you are speaking about a horizontal recess and the panel is mounted below the block wall. Most AHJ will allow 8" before they consider overhead being an interference. 

Hope this helps