Our tank goes all the way with bottom darkness, having a pure black substrate.
The stuff we have though is too sharp-edged for corys. They like mud, fine soft sand or very well worn stream gravel that has weathered for years and has no sharp edges. This keeps their barbels from being torn and scratched. When I put my hand in different gravels, I find that some can be quite smooth, whereas other types can come close to cutting my fingers.
~~waterdrop~~