There have been remains found in Fairford in Gloucestershire dating about 900-1200AD of that of a black woman in her 40’s (I think) probably a slave. Black folk in medieval England were more common than most folk think.
Yes I grant you that there was indeed the possibility of slavery, but more bonded labour as this was the feudal era.99% of bonded slaves would be white. A black slave would be something exotic, and not be as common as you suggest, a rare sight, and then only in the company of a lord or merchant class, although having a black manservent didn’t become fashionable until the 17th century.
A single skeleton of someone who died are the very old age of 40+(old indeed in the poor classes, unless food was good and that again points to the owner being wealthy, and the ‘staff’ reasonably well cared for. I expect where the remains were found would play a part in determining who her servitude was with.
All tha said, with the portrayal being a Muslim, that would play out, in the role of a friend to a crusader, as Africa beyond the north was quite uncharted, and slavery a common practice int he Ottoman Empire. Someone from abroad in that role would likely be of similar portrayal, but the same horror and shock reaction from the public would remain, because black or people from outside the UK or even Europe would be very very uncommon in the 12th or 13th century.