Since no one responded to this I'll tell you as a non-expert that Sudan was Christian far longer than it was Muslim. They rebuffed the Ottoman Turks in battle in the 16th century, but the Ottomans still settled in the area and gradually spread Islam throughout the region. Still, more African areas of Sudan, e.g. Equatoria, Darfur, stayed Christian, and remain Christian to this day.

Also, just want to point out that Egyptian Coptics are native to Egypt. They are not transplants. The, now, Arab world was very Christian before it was Muslim.

I'm not an expert on the subject, but I live in Sudan, know many Coptics, some Coptic churches, and read on the history of Sudan often. Currently I'm reading Pakenham's "Scramble for Africa".