I'm surprised no one has complained or brought this up before, because I think it's been going on since the game had that huge lighting change.
To put it simply, black hair isn't black. At least when you use the more natural hair options.
There aren't a lot of hairstyles to begin with, let alone black hairstyles, but I do like the dreads. The problem is that even though I have the color selected as black, it shows up in-game as gray. In the screen shot I provided, you can see how not black my character's hair is. It almost looks illuminated. There's a stark contrast between how it looks now vs how it looked in D1 and early D2. Hell, even in the screen shot, there's a huge difference between my black colored hair and Crow's hair.
On other hairstyles black looks fine. It's glaringly obvious once you use a natural hairstyle. If I wanted gray, I'd have chosen gray.