Carbon black would work, but you don't want it next to the skin. You might paint on a coat of regular epoxy across the area, let it cure, then mix carbon black, micro-balloons, and epoxy as a filler in that area. That works on the inside.
Alternately, you can lay down one layer of thin regular fiberglass on the outside, let it cure, then overlay it with carbon fiber and carbon black tinted epoxy. Use a top layer of regular cloth, fill and sand.
I did both.
