The hair colour of Legolas is never stated in the books, but as his father was a Sindarin elf and his mother (I believe) a Silvan one, he almost certainly had fair hair and I have always imagined him that way.

Only the High Elves tended to have dark hair and they were often described as the most lordly and wise.

They made Boromir fair haired and he betrayed the Fellowship. So, really, there isn't much of a case to be made for some kind of fair-hair conspiracy.