Probably sub-saharan africans would be a pretty normal sight in the mediteranean, either as rowing slaves brought by muslim merchants, or maybe even as Malinese merchants. The Malinese kingdom, during the time of Mansa Moussa only two centuries later, was one of the main suppliers of gold to medieval europe.

I guess in the medieval state of mind, the world would seem so large and ingraspable, with stories full of monsters and one-legged people on the edges of earth, they wouldn't be so surprised when facing someone with such a different skin tone. The spanish themselves would probably look very foreign and weird to the Londoner as well, with different styles of clothing, maybe beards, you name it, with spain being for the most part a muslim empire in the 11th century.

Racism is something of the 19th century.