I think you would need a special aluminium primer to paint it, otherwise anodizing is probably the best way to make it black although it would need to be sent off somewhere for the process to be done.
If you do paint it take your time, strip the case down as much as possible and paint it somewhere away from children, animals, dust and wind (preferably) just so you get the best finish without anyone knocking it. I think from what I've read the best way to paint is to go from one side to the other starting from bottom to top covering the surface with an even thin coat, of which you should do 4 - 5 coats for a decent finish.