This, of course, is not true. There was, for instance, the civilization of Kush, which actually conquered Egypt for around 75 years.
Unfortunately, since even anthropologists are blinded by their own prejudices, people simply ignored the existence of African civilizations. Or, if they did recognize their existence, they would claim that they must have been really been white, or founded by whites (a claim we've seen before). From the latest National Geographic (linked to, above):
Explorers who arrived at the central stretch of the Nile River excitedly reported the discovery of elegant temples and pyramids--the ruins of an ancient civilization called Kush. Some, like the Italian doctor Giuseppe Ferlini--who lopped off the top of at least one Nubian pyramid, inspiring others to do the same--hoped to find treasure beneath. The Prussian archaeologist Richard Lepsius had more studious intentions, but he ended up doing damage of his own by concluding that the Kushites surely "belonged to the Caucasian race."
Even famed Harvard Egyptologist George Reisner--whose discoveries between 1916 and 1919 offered the first archaeological evidence of Nubian kings who ruled over Egypt--besmirched his own findings by insisting that black Africans could not possibly have constructed the monuments he was excavating. He believed that Nubia's leaders, including Piye, were light-skinned Egypto-Libyans who ruled over the primitive Africans. That their moment of greatness was so fleeting, he suggested, must be a consequence of the same leaders intermarrying with the "negroid elements."
For decades, many historians flip-flopped: Either the Kushite pharaohs were actually "white," or they were bumblers, their civilization a derivative offshoot of true Egyptian culture. ...
--Robert Draper, "Black Pharaohs", 213 National Geographic 2, February 2008, pp. 38-39
It's easy to overlook the existence of black civilizations if you start off by assuming they don't exist.