Like. There are a lot of things I could say here. But I’m just going to do my best to answer your question, and the answer is either very simple or very complicated, depending on your current point of view.
1. “They” knew what people with brown skin looked like because people with brown skin had been there literally THE ENTIRE TIME. Some (and father back, ALL) of “them” had brown skin themselves.
2. “People with Brown Skin” and “Europeans” are not separate and mutually exclusive groups.
3. No matter how far back you go, the mythical time that you’re looking for, when all-white, racially and culturally isolated Europe was “real”, will continue to recede from your grasp until it winkles out the like imaginary place it is.
We can just keep going back. In every area, from all walks of life, rich and poor, kings and peasants, artists and iconoclasts, before there were countries and continents, before there were white people.
The time when “EVERYONE” in Europe was White does not exist. They knew what people with brown skin looked like because they were there. They knew what “Africans” looked like because they were there, and they weren’t “they”, they were us, or you. I think what you’re missing is something that never existed.