I think many taxidermists paint the horns black "to make them look better," and to cover up mistakes. When I shot the buffalo in my avatar, the horns were grey and polished smooth. When I got his mount back from the taxidermist, the horns were cracked, very rough, and black. I think the roughness and cracks were from over boiling them in Zimbabwe. I now tell my taxidermist NOT to paint the horns, and they look more natural.