I actually found black to show stuff more than a mix. I now use play sand which is a mix of black/gray/white/tan particles and the corys are doing very well. This also replicates their natural habitat sand.
I had always wanted a black substrate, so I set one up two years ago using Flourite Black. It always looked "dirty" so two weeks ago I pulled it out and replaced it with play sand.
The black also looked odd, perhaps because of the tank light spectrum; the sand looks different under water with the light too, though still natural.