Jennifer Raff has an article in the Guardian, discussing why there’s no genetic sign of black African ancestry ( from Roman times) in contemporary British DNA. Well, the simple explanation is very few such people ever came to Britain. She mentions the case of the Norse Colony on Newfoundland at L’Anse aux Meadows in Newfoundland, which also doesn’t seem to have left any genetic trace.
In the Newfie case, I’m fairly sure why there’s no genetic trace, having read the Greenland Saga. The Norse colony ( which was small) only lasted for a couple of years: the Skraelings turned hostile & the Norse apparently fought among themselves [Freydis]. Moreover, even if the Norse did manage to get it on with the local Amerindians [the Beothuk], the Beothuks’ extinction around 1830 makes genetic transmission to the present day most unlikely.