Black is a natural hair color, and Blood Elves have black hair as an option. Does that mean Void Elves can't have black hair?
White hair is also a natural hair color, and Blood Elves also have white hair as an option, does that mean Void Elves can't have white hair?
Blue hair is not a natural hair color (at least not in the human spectrum), yet Blood Elves now have multiple blue hair options. Does that mean Void Elves can't get any new shades of blue hair?
Purple is also not a natural hair color (at least not in the human spectrum), yet Blood Elves recently got a dark purple hair option. Does that mean Void Elves can't get any new shades of purple hair?
I think the devs would find the above as ridiculous as I do, and I don't believe they are going outright deny Void Elves options based on what Blood Elves have. That's not to say the devs will just copy all of the Blood Elf hair colors to Void Elves. I don't expect them to do that. But I do think some natural hair colors may be added to Void Elves in the future. Perhaps not the exact same shades that Blood Elves have, but I think they will happen in some form.
For example, I saw this picture recently on the official forums. I could see these being added to Void Elves as hair color options:
They aren't the full gamut of natural hair colors, but four out of five of them fall under what most would consider "natural" (White, Black, Blonde, Brown). The blonde option shown here is also nothing like the blondes that Blood Elves have (it falls more into the beige blonde category). I could see the devs adding something like that as opposed to giving Void Elves a copy of Blood Elf blondes.
I will admit some bias on my part because I truly desire black hair as an option, and black falls into the natural spectrum of human hair colors. And if an exception is made for black and white, then why wouldn't other exceptions be made as well?
I do think that Blizzard should give Blood Elves variants of the Void Elf hair styles. The one in my avatar picture is quite popular and I know several Blood Elf players who wish they had it as an option. I also think Blood Elves need more options in general such as scars, tattoos, and jewelry for the males, as well as the Dark Ranger/San'layn skin and eyes.
I'm hopeful that Void Elves get some natural hair colors so my character can finally look the way I envision her:
(black hair courtesy of photoshop)