Well, various areas in the Americas became African-descended majority, due to the Atlantic slave trade. Granted, the destination for agriculture in relatively low density areas in the Americas can be different from Europe, and Europe didn't ever seem to have a large population of African slaves within Europe. But maybe something could have happened, like with Italian merchant republics trying to establish plantations in the Aegean or Crimea?
Axum/Ethiopia in Arabia also sounds possible.
As for Portugal and Cabo Verde, maybe in a bit of a different direction, something with Lusotropicanism? Perhaps a scenario where the United Kingdom of Portugal, Brazil, and the Algarves survives can be one where migration from Brazil and the African colonies (which perhaps themselves end up integrated into the political system) leads to gradual migration patterns that leave mainland European Portugal at least minority-majority with African and mixed populations together outnumbering the white population, perhaps with some opposition at least for a time from the European Portuguese, but with European Portugal increasingly dominated by a Brazil that doesn't really care.
This would probably be more unrealistic, and is outside the bounds of the OP, but maybe if the 1975 Carnation Revolution went differently, a radical leftist Portugal might seek to integrate the colonies and integrate FRELIMO, MPLA, and PAIGC into the revolutionary government. Then, a mixture of migration from Africa to Portugal and vice versa, as well as intermarriage and such, and perhaps some refugees from various African and Middle Eastern Wars in the 1990s and 2000s, could lead to an African majority some time in the 21st century, if the revolutionary regime isn't overthrown.
Axum/Ethiopia in Arabia also sounds possible.
As for Portugal and Cabo Verde, maybe in a bit of a different direction, something with Lusotropicanism? Perhaps a scenario where the United Kingdom of Portugal, Brazil, and the Algarves survives can be one where migration from Brazil and the African colonies (which perhaps themselves end up integrated into the political system) leads to gradual migration patterns that leave mainland European Portugal at least minority-majority with African and mixed populations together outnumbering the white population, perhaps with some opposition at least for a time from the European Portuguese, but with European Portugal increasingly dominated by a Brazil that doesn't really care.
This would probably be more unrealistic, and is outside the bounds of the OP, but maybe if the 1975 Carnation Revolution went differently, a radical leftist Portugal might seek to integrate the colonies and integrate FRELIMO, MPLA, and PAIGC into the revolutionary government. Then, a mixture of migration from Africa to Portugal and vice versa, as well as intermarriage and such, and perhaps some refugees from various African and Middle Eastern Wars in the 1990s and 2000s, could lead to an African majority some time in the 21st century, if the revolutionary regime isn't overthrown.