What the hell causes this? Just got to this guy tonight and it really annoys me =/

It's a shader bug.
Go to Video Options and turn off Smooth Shading and maybe even Vertex Animation Shaders. I got the same problem with my Stygian WSG flag shield and it wasn't fixed until I disabled Vertex Animation Shaders, Smooth Shading wasn't enough to fix it.

The devs thought making him harder to see would balance your blessing advantage.

My gun from AQ20 is also black... For maybe 90% of all players... I heard something about shaders, but turning that off did jack. This is about as high on their priority list as fixing thaddius lag.

Same bug that makes Thunderfury black since ages ago.

I had this issue; I didn't change any settings and just updated my drivers, and it fixed it.

Turning off Smooth Shading did the trick for me. The game doesn't prompt for a restart but it is required. Both Thunderfury and Anub now look fine for me while they were full white/black before. Just make sure you restart before drawing the conclusion it doesn't work.