I'm not disagreeing, just providing (I hope) interesting context.

Black Nazis

At the time, there were a few Afro-Germans, obviously hated by the nazis. A few were children of individual travelers, such as diplomats, like Hans-Jürgen Massaquoi. More were children of French Colonial soldiers. They were denigrated as "Rhineland bastards" by racists/nationalists even before the nazis rose. Obviously, they were prosecuted; subjected to forced sterilization. Perhaps surprisingly, they were never systematically murdered.

There could not be such a thing as a black nazi. However, a few "mixed race" children (African and Asian) were conscripted into the Wehrmacht.

One should also bear in mind that the term Aryan existed before the nazis and was used by white supremacists in the US even pre WW1. The nazis stopped using it in the last years of their rule.

black vikings

Black vikings are historically quite possible. In Western Europe and the English-speaking world, the vikings are known as raiders and conquerors. But they were also merchants and mercenaries. An archeological dig, near today's Stockholm, found a viking age Buddha statue from India. "Vikings" served as the Varangian Guard in today's Istanbul. People also travelled the other way, of course, such as notably Ahmad ibn Fadlan.

There's nothing particularly implausible about a dark-skinned African travelling north to the Bosporus, meeting vikings, and travelling with them to their homeland to go on raids. It's not like we would expect to know if this happened once or twice during the viking age.

Arthurian Britain

I don't know much about this period. It was basically just after the Roman presence had ended. The Roman Empire included northern Africa, so thinking of black people in the Roman Legion makes perfect sense. After the decline/fall of the empire and the disruption of trade routes, the presence of dark skinned people in Britain becomes rather less plausible.