If you really
want it to turn black, mix up a scoop of Oxi-clean in enough water to fill it and let it sit over night.
I scrubbed my aluminum pot for hours and can't get rid of that little mistake.
Seriously though, you should be fine. The aluminum oxidation will be a dull grey colour (vaguely the color of my avatar) and not black. Sounds like yours was already aged.
Edit: for anyone who is horrified that I use a black aluminum pot- I figure if my best efforts with copper wool and a scotchbrite pad can't get it off, nothing short of sandblasting will. Boiling wort certainly hasn't removed anything.