"But I honestly can't think of one black supremacy movement since the late 60's."
Let me help you think. There's the Black Hebrew Israelites. Nation of Islam is still around. New Black Panthers. Any black gang in or out of prison. That's just off the top.

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Now, lets see the white ones
http://en.wikipedia.org/wiki/Category:White_supremacist_groups_in_the_United_States
And thats just in the USA. Add about 2 or 3 thousand more for Europe alone.
And Black gangs aren't Supremacy Groups because their primary focus isn't race. In fact, many majority black gangs (like the blood, crips, vice lords etc) actually have white members, along with people of any other race you can think of.