While some firearms have aluminum parts anodized, many have the trigger guard and bottom metal painted with some type of black enamel. It's very easy to recreate the original look with a bit of fine sanding and a spray can of Krylon. I've used the semi-gloss a few times, and think it gives a better finish than the gloss stuff. Of course the Krylon is nowhere near as tough as anodizing, or the spray and bake finishes, but it is very easy and inexpensive to refinish again whenever it's needed.
Another option that is pretty durable is to have it powder coated ... a variation on the spray and bake thing.