Aren't the dark skinned people Egyptians rather than Africans? There was
a great deal of interest in Germany, Austria, and France in those days
about archaeology discoveries in Egypt and the middle east...  and there
were a lot of travelers there from the middle east...  This was a new and
exciting time for the Europeans resulting in a lot of music and writing
that explored the subjects...  I don't know the details.  I'm just
speculating...  I know that some of the characters in the Opera are
supposed to be Egyptians or middle east people of some ilk.

Ray Bayles