Rich,
Personally, I would start with a Rit Dark brown and add some black to it. Be sure you are adding a small amount of vineagar to it so that the dye will set. If you look at many old horns, the tips dont appear black but a very dark brown. To add age, and after the dye has set, I use some #0000 steel wool to rub off areas of wear. If you want black, then you have to add some dark brown to tone down the black/purple tint. The longer the horn sits in the dye the less purple tint will remain & be sure to use vineagar. Hope it makes sense.