Oh fun history time!
Especially a couple thousand years ago, the Sahara was far smaller, and far lest hostile to life. The last trees only died in the Sahara about 20 or 30 years ago. Caravans crossed regularly. On top of the very easy route from Sudan to Egypt, and the Greeks were obsessed with Egypt. Even the Bible talks about Nubians in Ancient Egypt, so if even that as a "source" knows about the well-documented reality that Sub-Saharan populations were in contact and well-known individuals present in North Africa and the Levant as far back as 5,000 years ago.
I mean, did you not even think to search for this before spouting off? Literally the first search result for "did black people greece":
Edit: meant to paste this link - my bad - https://en.wikipedia.org/wiki/African_Greeks https://en.wikipedia.org/wiki/Black_Greeks