If you're driving on roads that have been sanded, it could be an accumulation of a lot of tiny rock chips. It's a bit better if you have running boards, but the same thing will happen to any color.
I parked mine to make sure that didn't happen. There's only one black one in the area, and his is a daily driver - the paint at the front of the rear fenders was pretty well gone after he made a couple of trips through the mountains, but the hood still looked pretty good - most of the damage was from sand churned up by the front wheels.
I took the plastic rear fender protectors off this fall - really cleaned up the look of the truck. I'm going to repaint them - look pretty good, but definitely some rock damage - and put them back on for long road trips.
Unfortunately, one of the downsides of owning a really unique, sculptured vehicle.
I've got a Duramax with running boards, and very little damage, even though it's constantly on bad roads in the winter.
Ray
