Believe it or not, its best to be vague, it actually helps readers connect more with the story on an emotional level. There has been a bunch of scholarship done over it lately, I wrote a paper over it in Psych and lit a few years ago. It has to do with being able to impress your vision of beauty/yourself on the characters or some such, essentially its the same reason why in a lot of hentai the male's faces are darkened out or shadowed. *insert more you know rainbow here*