That’s a rather confusing question. How can you write what in a way that fits your story? I mean… it’s a fantasy story where real world ethnicities don’t actually exist right? I mean, blond east-asian is pretty much self contradictory unless they’re also albino, but there is no asia in this fantasy world so what does it matter. On the subject of science, well, I don’t really understand why you keep bringing it up. Realistically, if someone has naturally blond hair and blue eyes then they aren’t monoracial East Asian genetically. Races are a combination of actual biological differences and artificial social constructs. They’re extremely subjective and semi arbitrary labels that humans created to differentiate groups of ourselves from other groups of ourselves based on the appearance of relatively superficial genetically inheritable environmental adaptations. So basically if you make someone who has traits that aren’t a part of a certain “race” then they just aren’t only that race.
It’s a little like dog breeds. Dog breeds exist because humans genetically modified dogs through breeding. We took the basic dog we had all those years ago and with our basic understanding of genetic inheritance at the time, artificially selected for traits that we wanted to promote and against the traits we wanted to discourage. Over time we created the different breeds, many of which have health problems due to our tampering. All the different breeds of humans(“Races”) are a the product of a similar process. Based on the environment, those individuals whose traits didn’t disadvantage their ability to breed, or gave them an advantage over others passed on their genes. The environment of human evolution of course includes what we generally consider “the environment” or “nature” but it also includes what we consider the trappings of civilization. It’s very complicated because humans have more power than any other species to alter their environment, which in turn alters them. But we’re still affected by the things we can’t affect.

I don’t know why I can never number things correctly.