There is also a sort of idealization regarding "what features do Westerners have that Asians don't that is found attractive" so the probability of an idealized character being blond(e) / blue eyes is very high. To augment >>82, Rose of Versailles is probably the work most responsible for cementing this archetype, especially as regards attractive androgynous female badasses (which the Nasuverse is absolutely infested with in case you didn't notice lol, but it's seriously an oft-cited influence to this day).

Many (certainly not all) haafu in anime are portrayed with that combo despite that a majority of Westerners aren't blonds and by animu logic they might not "actually" be blond just like their classmate might not "actually" have pink hair. (When we're going with idol/mahoushoujo crap, yellow-themed girls tend to be the onee-san type, for example, so it may not have anything to do with Western genes). High school genetics even claims it's impossible barring a very unlikely mutation--that's not the whole picture, but let's just say I've never heard of either trait occurring in someone who has, for sure, a 100% Asian and a natural blond or blue-eyed parent; granted, I don't look very hard, but I do live (and grew up) in one of the areas they are most likely to exist.