The Bug:

The Black (Tuxedo) Cats have practically no shading on its texture; nearly every single bit of it is one solid color. This is inconsistent with how all of the other cats, even the other All Black cat, is shaded, and with how Minecraft mobs in the vanilla game are shaded in general. 

Both of these renders are completely unshaded from lighting effects; its the pure, raw texture. The difference in texture can be seen very clearly here. All other cat textures have some sort of shading as well.

It has been like this since the texture update, before that the programmer art Tux had a bit more shading as well. I can only assume it was a mistake or unfinished/overlooked