I'd be shocked if they put that much thought into it. It's probably just good old unconscious ethnocentrism. Jesus is white because I'm white and that's my default setting to view the world.
There's quite a bit of evidence they knew what other people looked like. Western Christianity goes big in Italy and they have regular trade contact with the Levant and Northern Africa.
That trade contact comes to the entirety of Europe thanks to the Byzantine Empire, also known as the Eastern Roman Empire.
If you like that sort of thing then there's a ton of philosopher stuff available online. You just really need to make sure of what they mean when they use certain words. Like, Locke was not talking about universal healthcare when he said a government has a duty to take care of the health and welfare of it's people. (But I did have a fun time arguing that he would have been in favor of it with my professor.)