While I agree with you, writing new characters to meet their race quota would just create different problems. If the character's too important, fans of the original will hate that character for effecting the story too much, but SJWs will hate it if it's not important enough. Blackwashing happens to be the simpler solution.
As the saying goes, you can't please everyone.
See, at this point I am really, really tired of this. Blackwashing and whitewashing are something that should be dropped completely. It's NOT how you reach diversity, it's not something progressive and you just piss off the fans. It's not ok to change the source material. It's not ok to change the real author's vision.
Because the author knows better.
Take the LotR/Hobbit movies. Every time they diverged from Tolkien's vision the result was subpar. Every time they followed the book to the letter the result was fantastic. One of the producers, in the extras of the CE, actually said something along the lines of "We tried to change things here and there, just to realize that Tolkien knew better".
But the main reason to avoid it is still THAT is the source material and your should not change it.
Also, a question: I don't follow GoT, did they go and made diversity for the sake of diversity with its characters?