JPUSC and chewchoo gave some examples of Black African saints.
However, some of the other Saints mentioned like St. Cyprian of Carthage and St. Augustine of Hippo were not black. They were North Africans. Black Africans come from Sub-Saharan Africa, while the inhabitants of North Africa (above the Sahara) were in ancient times and still today Caucasians.
For example, St. Cyprian was either of Punic (Phoenician) or Berber stock. St. Augustine was a Numidian Berber. The Berbers are the original inhabitants of North Africa and are very interesting people. You should read up on them; they have quite a fascinating history. They are Caucasians, though, so St. Cyprian and St. Augustine would not be examples of Black saints. Remember: Africa does not always mean black; it’s just a geographic location.
Hope this helps.