Yea I've seen those NPC's and you can choose the black hair when using the wow model view program. But the texture isn't finished. It doesn't have the coloration for things like the hair band for pony tails or the hair tentacles.
Here's my character in the model viewer using the void elf black hair option:
(the model viewer doesn't show the eyeglow properly but those
are the "arcane" eye option we have in the game)
Here's one of the void elf hairstyles that has both a hair band and hair tentacle:
Granted in the game, the greyed out tentacles look like solid black tubes and not the grey with white outline in the above image.
I'm hoping that Blizzard finishes that texture, and adds a stark white, ashy beige, dark brown, and perhaps a really dark brick red hair color as well. I want more variety. I'm not concerned about High Elves. I just want my void elf to look the way I want to look and I don't really care what the lore has to say about it.
The skin tones were billed as "High Elf Customization" by the developers themselves. So if "High Elf Customization" is a thing (and it is) then I see no reason why it can't include hair colors.