Yes, black Africans served in Rome's Legions!
There is every likelihood that black Africans did serve in Rome's legions at various stages, and were not just auxilliaries (Numidian cavalry) or servants, as troops were recruited from all across the global Empire, and anyway Rome didn't have the same hang-ups about race as Westerners do today.
Black people have been archaeologically proven to have been members amidst Roman Britain's high society, for example- wealthy and influential, so why not as soldiers?
By the 2ndC ad the majority of Rome's legions were not even Italian/Romans.
I think that the white supremacists and/or extremist idiots have been foaming at the mouth over the mere possibility?