One should never use oxyclean or any other activated oxygen cleaner on your aluminum pots. Aluminum oxidizes rapidly and the result is, well... that blackened mess you found. It is one of the very few disadvantages of using an aluminum pot.
Chances are the pot is not ruined, but you will need some elbow grease to scrub off all the blackened, sooty 'film' that you can. Once it is clean again, just boil some clean water in it for 30 mins to restore the thin al-oxide layer that is desirable.