I just recently tried this http://www.woodcraft.com/Product/2004505/8370/System-Three-Epoxy-Pigment-Black-2—oz.aspx which seems to work well.
But always used artist charcoal before. I bought a pack at walmart for a couple of bucks, and use a utility knife blade to grind a little up and add to the epoxy. Its a pain for a large amount, but works well for small repairs.
-- There is nothing like the sound of a well tuned hand plane. - https://timetestedtools.net and https://diy.timetestedtools.net