Do you mean that we thought they were black up until recent history? Or do you mean they were black up until recent history?
The idea that the Celts, and vikings were black is largely thrown out by most historians, because it lacks proper evidence.
The man claiming Vikings were black is claiming this based off the Saxons referring to the Norse as “black devils”. This is not at all sufficient evidence.
Well, sure there are some racist scientists, but i do not buy that there is a racist cover up of history.