I've been using an AL pot as my brew kettle for over a year and there is no black coating. There is some minor discoloration that occurred due to use, but no discoloration. The only time I have seen a black residue is when I accidentally left and oxidizing cleaner solution in overnight.
To remove, I boiled some water with cream of tartar and it worked great! No scrubbing, just let it boil for 30 mins maybe... i do not remember. A page that I found off google said to scrub with a cream of tartar paste for the same effect.