Do NOT scrub the inside of an aluminum brew pot!!!
Boiling water in an aluminum brew pot will build up an oxidation layer on the aluminum, which prevents the aluminum from leaching into your beer. That's likely the black coloration you're seeing, and you WANT that!
The inside of an aluminum brew pot should be gently washed only - make sure any big chunks of stuff is out of there, make sure it looks clean - but never scrub!