It IS normal to chisel into concrete blocks to install electrical boxes. The boxes used are called concrete boxes and are made for the purpose. They are interconnected using emt conduit.
Anyway you want to fix the block. Regular mortar repair will just fall out. Hydraulic cement expands when it sets up - usually in about 20 minutes. Ever see one of those silly putty eggs? That's the consistency you want to make up with the hydraulic cement. Put it in there little at a time and build up from the bottom.