Darryl has it right with the dyed Accraglas. You'll want the liquid, not the gel, and I'd recommend using the Accraglas dyes (Brown & black) as well. If your horn is jet black, you won't need the brown.....otherwise you can generally mix the two colors to get a match. After 24 hrs. of curing, sand the area lightly with 320 grit wet or dry and progress through 400 and 600 grits as well. To polish, hit it lightly with 0000 steel wool and apply a drop or two of your stock finish oil rubbed in vigorously by hand. That's it.
Robt.