Hey mate take some 800 or so grit sandpaper to it just to rough it up slightly and "open" the aluminium up.
Buy a small bottle of birchwood casey aluminium black.
Either wear a mask or use in a VERY well ventilated area (ie outside).
Apply using a q tip/cotton earbud.
Apply a coat and kind of swirl it around at the same time.
After that coat has dried apply another. Put on as many or as few coats as you like for personal preference. I like using 3 or 4.
If you would like it to be a good deep rich dark black black dip a q tip into some black ink (i use printer ink lol) and rub that over the weathered area.
The weathering from the alum black essentially "absorbs" the ink.
Allow 24 hours to dry then put a clearcoat over it.
Don't cheap out and use crappy clearcoat. Get a $10-$15 can of either flat or low gloss clear.
Remember with your clearcoat to apply very thinly, dry, then very thinly again.
Don't try doing it thick and all in one go, it doesn't end well.
The result?
Some nice black weathering