The black pistons are made for smoother tracks. Found this quote in the Inferno MP9 thread:
"The stock angled pistons are really made for smooth tracks with bigger jumps, that need alot of pack and little rebound."

Another:
"White is tapered with straight holes.

Black is tapered with holes drilled at a 45 degree angle.

Get the whites. Black slow down the shock compression and rebound since the holes are drilled at an angle. I have a set of both and I didn't like the black pistons when I tried them. They would probably work better for fast smooth tracks."