What tribe are black people descended from?

Among the so-called "black-skinned" people of the world, there are actually many shades of skin color from light tan to deep black. Dark-skinned people who live in the Western world (such as African-Americans), the Caribbean, and other places usually are not identified with particular tribes that their ancestors may have come from. Since there are hundreds of different tribes of dark-skinned people in Africa, not to mention Australia and Asia, this question really cannot be answered. Skin color is not a useful way of categorizing ethnic groups of the world, and many people would find this question to be racist in its assumption that all dark-skinned people are tribal.