From outside, probably not. The Nubians are less numerous and politically quite divided. Not to mention the largest Nubian state, Makuria, is a Fatimid ally.
Nubians comprised a large part of the Fatimid army, though, especially Nubian slaves. I could see a Nubian soldier rising through the ranks and potentially staging a coup (possibly motivated by fear of Salahaddin or someone else who would be less inclined to maintain the alliance with Makuria). If that Nubian coup leader happens to still be Christian... Well, the Crusades just got a lot more interesting...