Hey OneFeather, I've done some carving on two black horns.
The first was a horn picked up in a yard sale. Shape of it makes it look like a water buffalo horn - narrow but round on the butt, flattened towards the tip. Anyway, mostly black with some shades of brown. Carved my name in wide block letters and "his horn" in narrow script using the point of a pocket knife. The "scored" horn turned whitish where the fibers were cut. Polishing it down or otherwise rubbing it smoothed out the cut and took some of that white out, but the letters still show nicely from 10-15' away without being garish. I'm sure they will fade over time as the surface is worn down and smoothed, but all I'll need to do is go over them again with the point of a knife.
The second horn is a bison horn - jet black. I've carved the surface of the horn down to leave the shape of a buffalo raised above the surface. Everywhere I carved, I later had to polish pretty good to get rid of the the whitish color that came with cutting the fiber of the horn. For the shaggy cape, I used a rasp to roughen up the surface of the polished horn and give it some texture... where it is rough, the low spots are still showing as an off-white color. There is a pic as it looked a while back posted here:
http://www.muzzleloadingforum.com/ubbthr...true#Post166059
You can see how it goes whitish where it is scraped, gouged, carved, etc.
I'll get another one up when it is finished, which should be Real Soon Now (tm). And, BTW, I figure this baby is PC to the first decade of the 22nd century, so don't waste your time on comments about "historical examples." :: (though it might make a good companion to a GPR)
In both cases, the carving on these black horns can easily be seen from across the room without being too overwhelming.
Give it a try with a small shallow cut (dull blade works better). If you don't like the looks of it, you can polish out the cut and it will never show.