Al Akhdam

"They are known as “Al Akhdam” — the servants. Set apart by their African features, they form a kind of hereditary caste at the very bottom of Yemen’s social ladder."

Very good article on the "Akhdam" of Yemen. I can personally vouch for the facts in there as I was associated in the late 80's with a project with the Akhdam in Sana'a.