I think the question about Arab slavery is a little more complex, while white (Caucasian) slaves often ended up as soldiers, administrators and concubines/wives and black slaves mostly as field slaves and house servants, at the same time Ethiopians were sought for their beauty [1]. The Muslims did make a very clear distinction between Africans from the African horn and other Africans. Eunuchs was also overrepresented among African slaves, a major reason for this was that some school of Islam banned Muslim from castrating people, so the procedure was often done by Ethiopian middlemen.
As for the question about mixed European and African descendent royalty and nobility. A major problem is that geography split these population from each other making the degree of mixing people suggest unlikely. But there is a solution, an earlier colonization of the Americas would result in stronger American aristocracy, and give more time for free black to rise up in the ranks. We could easily imagine that a Vinlandic population which have spread to the Caribbean and began to import African slaves through the Portuguese (who would likely establish a trade route across the Atlantic around 1400, if they knew there land and people there), would see the rise of biracial or triracial aristocrats, and they could over time even become the dominant group of the Caribbean (they would likely look like Dominicans), these people growing rich from their estates we could easily see intermarry with the mainland white Vinlanders, as their riches would make their mainland cousins ignore racists or classist concern over their dark skin or illegitimate origin, while the marrying the mainlanders would be important as the islands likely dependent on food imports.
[1] At least two Omani Sultans were son of Ethiopian concubines (the Omani Sultans great and great great grandfathers)
