Did you miss the "popular" bit? It means he was renowned enough that normal medieval folk would at least have
heard of black people.And in a society largely illiterate, the positive depiction of a black man in art and in your churches means
everything.
Of course it's not just "one saint", don't blame
@Mila for you not looking it up yourself: one of the Biblical Magi,
Balthazar, was frequently depicted as a black man in medieval art. Likewise, you have
Saint Benedict the Moor, who is even more explicit.
But let's say you don't accept Saint Benedict as an example since he lived and was canonized centuries after the year KCD takes place in, 1403. Let's say I have to refrain from mentionning events later than 1403, like the Christian
Emperor of Ethiopia who
sent a delegations of Coptic priests to the Council of Florence in 1441, and who was confounded with the mythic Prester John, a foreign and fantastic Christian king in pagan lands.
Well then, I still have the fact that we have
concrete proof of contacts between Crusaders and Christian Nubians in the 13th century:
Robert de Clari's full text
here.
And if you say that this is taking place far from regular European medieval folks, I can always point to closer things, like
a 13th century chivalric romance from the Arthurian cycle about a black knight looking for his deadbeat father:
(and yes a black dude is described as handsome in a medieval story, weird huh)
Anyone interested can read the rest
here. This was popular at least in the Low Countries.
And
Morien is not the only black character in Arthurian legend, you also have
Feirefiz, half brother of Percival in
Parzival, one of the most popular story in medieval Germany. Feirefiz, despite being half-black and with a magpie-like skin, is the equal of Percival in prowress and described as very handsome.
I could go on but I think you get the point: the idea of black people was not completely alien to medieval Europeans, and, unless the Czechs were as completely isolated from the rest of the world as were the Australian Aborigenes, they should have the idea of it. Even better, as shown in my historical examples above, contriving a reason for black people to show up shouldn't be
that hard if you bother to look up African-European contacts before the Atlantic slave trade.
As an added bonus, I just demonstrated that lily white medieval Europeans had no problem at all to insert black people into their works of fiction, and in a positive way to boot
1. That in the Year of Our Lord 2018 people are still against the idea under the pretext of "historical accuracy/realism" says
a lot more about us than it does our forebears.
(well not mine, my forebears were in Africa, but you get what I mean)
1Not that there weren't bad depictions,
The King of Tars come to mind.