@The Dragon Demands I watched your recent 1hr and 15 min video on having a black actor for Corlys.
I have a few counter-arguments, but too lengthy to write in youtube comments. Let me preface that I agree this is a minor change and that you make several excellent points. I also don't have a problem with Corlys being black, but I do have a quibble about his appearance and that of his children, which I'll bring later. My arguments here are about some of your arguments. A few times your supporting evidence wasn't that strong, so here goes:
1) Valyria was a melting pot of peoples and the Free Cities are mixed places. You've mentioned this a lot of times in the early part of the video, that the FCs are mixed, so it's okay to have a lesser noble house like the Velaryons search more widely for marriage partners.
Valyria was a melting pot of slaves oppressed by a society that looked down on many others. Yes, the dragonlords were obsessed with blood purity, but that doesn't necessarily mean other noble families were fine with mixing with other peoples. You mention Pentos, but Pentos was not really founded by nobles: From TWOIAF - The first Pentoshi were merchants, traders, seafarers, and farmers, with few of high birth amongst them; perhaps for this reason, they were less protective of their Valyrian blood and more willing to breed with the original inhabitants of the lands they ruled. So from this sentence, it's clear noble families had a tendency to be protective of their lineage. While the Velaryons are a lesser noble house, they are still nobility. And nobles tend to look upwards more often than elsewhere, which in the case of Valryian families would mean looking towards the dragonlord families.
Pentos also brings me to my other point about what you said about the Free Cities being mixed. This is too much of a generalization, and it does a disservice to the true mixed city of Braavos, and yes Pentos, too. But you can't say Lys is that mixed where more than anywhere else in the known world the old Valryian bloodlines still run strong, or Volantis where the ruling class is made up of only those of Valyrian ancestry, or Myr who are homogenous enough to make people believe they are related to the Rhoynar. Qohor and Norvos were founded by religious cults and seem to have a caste system where the only melting pots may be in the lower classes.
2) The Velaryons being seafarers and merchants would have the opportunity to mix with other people, like Summer Islanders. True, but they are not the only ones. Oldtown is a far older and even better known port to even distant merchants than Driftmark is. Summer Islanders call there all the time, too. Why aren't the Hightowers more mixed? Well, the answer has to do with the conservative nature of Westeros high society. Marriages are primarily done for political gain. There would be little political incentive for a Westerosi house to allow one of their members to marry a Summer Islander, or someone else from really distant lands. And while the Velaryons have Valyrian lineage, in the centuries since they settled on Driftmark, they have become a Westerosi house, adopting the Faith of the Seven, knightly traditions etc. So wouldn't the Velaryons also look for marriage partners among the other Westerosi nobles? Yeah sure, there is the occasional marriage for love angle, but without more information about Corlys, that cannot be an argument's basis. Prior to Corlys the wealth of House Velaryon came from them profiteering of the Blackwater Bay trade, so tariffs likely. So again, the Velaryons would have looked closer to home to assure the continuation of their house.
3) This universe is like medieval Europe, and medieval Europeans didn't have the same views on race as modern people.
This is your best point, I totally agree. That being said, there are few tidbits here and there in the books (novels, TWoIaF, I think F&B too) where Westerosi react with fright when they see a Summer Islander. Jeyne Poole when she sees Jalabhar Xho. Ooo, but maybe D+D remembered that one line which is why Missandei and Grey Worm got the dirty looks from the northerners as you mention in the video. I'm not defending D+D, yuck, no!
So do some Westerosi react this way out of racism or ignorance? If it's the former, then maybe we can't equate TWoIaF with medieval Europe. If it's the latter (likely) then this hints that the presence of black people in Westeros is extremely rare.
4) So my quibble about how Corlys looks and from the leaked pics his daughter, too, is the white hair, the apparent Valyrian hair. Why? Is this their way to say hey this guy is not just some black guy, he's a Valyrian. Ok, but both books and GoT point out that when two distinct looking people have children, the one's whose 'seed is stronger' pretty much gives the hair color to the children. That seems to be the first of a person's features that is inherited from the dominant gene. I don't have any medical knowledge to know if this is usually the case in the real world, but it seems to be in this world. Remember Ned's recitation about the Baratheons. (and the show didn't even bother including eye color) Now we don't know how Rhaeneys looks in the show. But if they follow the book on this one, then I would find it very odd if Laena and Laenor also have the silver-white hair, just because their dad has it, who is presumably half Summer Islander. (who have black hair)
To conclude, my arguments have drawn from book lore, and it was mainly to point out some issues with your arguments. But thanks to GoT, which ironically is helpful here, the show universe has its own lore that is only loosely related to the book lore, and the writers have the freedom to craft their own consistencies. I just hope that there will be consistencies.