Have you tried PreHeat? Aluminum is real touchy about preheat.
It is a very good Heat Sink and will rob the heat from your Arc.
Many years ago we would lite the Oxy-Acet torch turn off the Oxy and cover the work area with the black soot then tune the torch back to a neutral flame and preheat until the soot burned off then it was ready to weld, but now days you can just go pick a 400 deg F temp stick or use a Pyrometer.
As for the Black have you cleaned the work? Another thing Aluminum is touchy about. Some Aluminum products have a coating to prevent oxidation, and then again some are not pure Aluminum.
If you look back thru some earlier posts on this Forum you find other discussions on Aluminum with lots of useful information.