I have had two black Vipers and find it is the best looking and classiest color but it must be kept clean. The best part is you will almost be forced to keep it clean since the dirt will show, but you will have the peice of mind your vehicle is well cared for. Remeber, no matter what color you own it will get dirty just the same. Some colors just "hide" the dirt better but it is still there.
I too thought about the slate, but realized I was only considering it because of the "looking good while dirty" concern. I went with the Black (for the second time) knowing I had to take better care of it and that the black look would pay off. Just put a decent coat of wax on it when new and use the spray wax such as NXT every few washes to prevent swirling. Scratches will show on any color, but swirls definatley show more on black. I hope this helps your dilemma.