I treat black top (Knock on wood!!) with power burst. However must try Blitz!!.
The way i treat it is to usually prespray said area at the start, agitate and then continue on cleaner areas. Come back to it then repeat if necessary (which it usually is!!!)
I was also told that mpower was the new Messiah on Black top. However recently cleaned a really dirty "domestic" kitchen carpet. Pre-sprayed with mpower more than normal m/s, agitated and let dwell for at least 20mins. Results were nothing short of abysmal. had to revert back to power burst and it came up a treat.
I must say that im a big fan of mpower on suites and certain carpets but it seems on black top it's pointless.
Will