Two methods that have worked well for me are using a black glaze wash, and also lamp black. The black glaze I get through Woodcraft, thin it a little with water, then brush on the carving. Before it dries, wipe off the excess with a rag. It'll stay in the recessed areas, and be wiped off the high spots.
For the lamp black, I slightly dampen the area around the carving with what ever finish you're using (generally I use Permalyn), then use fingers to smear lamp black into the nooks and crannies. Again, you can use a rag, or your fingers to feather out the edges, and get it off the high areas.
Ed