You might try this trick I recently learned from a MA.
Try layering on a deep color, but not black, underneath, and then lay the black/carbon over the top.
For example, I did some red and black eyes, and I put down sketch where I wanted the darkness, and then tapped the carbon over the top - this way the color stayed "true", and didn't look muddy.
it works great - laying down a dark shade before the black really helps prevent the black from getting muddy/messy or blending too much into the other shade.