So this is kind of a complicated question. The concept of black knights was not unknown to medieval people within their art. There's a few routes to go on this one, but I'm going to start with Saint Maurice. Here's a statue of him: https://upload.wikimedia.org/wikipedia/commons/a/a7/Saint_Maurice_Magdeburg.jpg

Now here's the thing Maurice was an Egyptian born Roman legionnaire. So he would not have looked like that. But he was pretty popular enough to be a patron saint of the Holy Roman Empire. And medieval artists had a habit of modernizing stories to fit with how they saw the world around them. They wanted to depict Maurice as a warrior, so he dressed in that days warrior garb, and looked a knight.

There were also several that existed in legends. The Knights of the Round Table had several "black moors" join them in their various adventures. Sir Morien was the son of one of Arthur's knights who traveled through Moorish territory and sired a son who would eventually go looking for him and join Arthur's court when he came of age.

Now it's worth pointing out that if Arthur existed at all he lived well before there even were knights. So this story is legend more than truth. But it does touch on what was going on in Europe at the time the legends were written down more than during the time of the legends were supposed to take place.

In 711 the Umayyad Caliphate from Northern Africa conquered most of the modern day Spain and Portugal. And they had their own forms of nobility and warrior elite. Though they eventually were pushed out of the peninsula, they held Grenada until 1492 so non-white European born nobility definitely existed.

As to knights specifically, there we come across some problems. if you want a specific person with a specific title I cannot give one to you. I can just say that in Spain and in the Crusader states there was constant mixing of ideas between the Europeans and the Arabs or Africans they dealt with on a daily basis. There are depictions of Europeans who dressed in Arabic styles, and Saracens who wore European armor. Back in Spain the Moriscos were the descendants of the Umayyad conquest and there is definite evidence that they took part in war and even had some Christianized nobility that served the crown.

However, tensions between them and the white rulers was pretty high, to put it lightly. And they were expelled from Spain in the 1600s. But that is a good solid chunk of time where non-Europeans were part of the social and military hierarchies of the time.

There are a few more stories of "black Moors" in Britain. Not surprisingly after being kicked out of Spain they showed up in the lands around Spain. In Britain there are a few snippets of them being honored guests in King James IV of Scotland's courts, and of course in Queen Elizabeth I of England lands, where she thought there were far too many on her land and wanted to be rid of them.

And that's about all I know, outside the odd unexplained black faces that occasionally appear in artwork in a military context. To sum up, non-white nobility definitely existed in territory that was once conquered by non-whites. If you are interested in a Christianized non-white who is part of the knightly class I can't give you any specifics though I'd be surprised if none ever existed. If you just want a non-white person who dressed in the style of armor we associate with knights. Then yeah, anywhere with trade to Europe. European armor was considered very good. Hell, there are depictions of warriors all the way in Japan who wore European breastplates they got their hands on.