If they're aluminum... there is a process that they call hard anodizing and it was originally developped for the military. It's black and it will stop the aluminum from being scratched by increasing surface hardness. It's also been used on aluminum sprockets, gears and pulleys for increased life. I really want to do it to my unorthodox racing pulleys. Anyway....
If they're steel they could receive a black oxide surface treatment.
Out of the two... the black oxide is the darker one but it's a flat black. The hard anodizing is not as dark of a black but the surface is really smooth and is more of a semi-gloss or gloss finish.