Cycling is not really popular among Africans and Arabs in France. And I don't believe in Eritrea's potential (or Rwanda). These countries seem to produce only pure climbers that underperform in time trials, like Quintana, which mean they won't be able to win a Tour.
However, the best potential for a black cyclist to emerge are the French Caribbeans IMO, particularly Guadeloupe. Cycling is pretty popular over there, they race and have a local Tour. France had black Caribbean cyclists in the past. A lot of speed cyclists at the Olympics are French Caribbeans.