Get a pressure washer, works great but don't use the jet tip on concrete, use the fan tip up close. The jet tip will etch the concrete if your not very careful. Hit it with Dawn before hand. Pressure washer is so good at cleaning blocks and heads: spray them with Gunk, let them sit for 10 minutes, scrub trouble spots with a tooth brush and then pressure wash it all off. Wear a face shield as it will splash back the black crap on you and all your clothes. Comes out very clean though.
