What do you want to achieve? Which of charcoal are you using? (Pencil, Willow, Vine...)
Don't know that much about charcoal but I'll try to help.
(Type of charcoal can be either vine and/or pencil. I have both.)
Mostly I'm trying to do away with the shine of the graphite where the darkest values are. (Shadows and what not.) Beyond that, I would like to achieve true black while doing so.
I know that graphite can get fairly dark, but after a certain point it won’t get any darker. Now I can use prisma’s black over graphite and get true black, but I still have a bit of shine. I would like to pull off both, but I guess one is better than neither if all else fails.