Nedarim, Chapter Three, Mishnah Eight
This mishnah teaches that one who vows not to benefit from the black-haired is forbidden to benefit from all men, but not from women and children.
He who vows [not to benefit] from the black-haired may not [benefit] from the bald or the gray-haired, but may [benefit] from women and children, because only men are called black-haired.
The word black-haired is interpreted to be a reference to men, whether or not they have black hair, gray hair or no hair at all. Women are not called black-haired because they typically covered their hair. They were called head-coverers. Children were called head revealers because they went without a head covering. However, men were called black-haired because sometimes they covered their heads and sometimes they did not. Furthermore, nearly all men had black hair. Indeed, the only other hair color referred to in the mishnah is gray. No blondes or brunettes in Palestine in the Mishnaic period.