This old thing. Well, if one reads the Greeks, one will find that they admired the Egyptians, which is a much older civilization. The Greeks, like everyone else of any sense, freely borrowed whatever ideas they found of value in other cultures that they encountered. So, some ideas the Greeks had were had previously by Egyptians and others. But, the Egyptians were of various races (as anyone can tell by looking at the various colors of the paintings that the Egyptians made), so their culture is not specifically "black culture" (nor is it "white culture" either). And more importantly, there is nothing in the ancient Egyptian literature that reaches the levels reached by the Greeks. In other words, the borrowings that the Greeks did was not the end of their contribution to civilization; they expanded old ideas and developed new ideas.
If we look at the roots of civilization, we will find them in various places, like Mesopotamia (modern day Iraq), Egypt, India, and China. In a sense, every culture since those civilizations started have borrowed ideas from one or more of these civilizations. But it would only be a "stolen" legacy if one denied such things, and that is something the Greeks did not deny. They never pretended to be the earliest civilization. As an example of something important the Greeks did, they brought mathematics to a higher level than it had been previously. But they did not invent mathematics, nor did they pretend that they invented it. But their bringing mathematics to a higher level is something that is important, and we have borrowed that higher level from the Greeks, and we have gone further still with mathematics (as a society, not that most people in modern society have gone further with mathematics than the Greeks).
So, there is some truth in the claim of a "stolen legacy", but it is a twisted version of the truth. It is also worth pointing out that what I state above is nothing more than what scholars have been saying for centuries. There is no vast conspiracy to deny the importance of Egypt for the development of civilization. And there has been no vast conspiracy to pretend that the ancient Egyptians were blond, blue-eyed nordic types. Think back when you were a student; in history classes in grade school, what were you told about the earliest civilizations? Surely, no one told you it was Greece that was the first civilization. And if they did, you must have gone to a very backward school.