Actually, Birchwood-Casey makes an aluminum blacking solution, it comes in a small plastic 3 oz bottle and sells for around 8 bucks. ( also comes in industrial sizes of 1 and 5 gallons for those BIG scratches)
I've used it in the past with mixed results, but it's good for covering small scratches and such - don't know how it would do on large areas . . . . as I recall it's a stop gap measure and not extremely durable.
A quick search on the net turned up places like
this that sell it; I think I used to order mine from Brownell's . . . .