-I don't actually know why, but it could be because they've read or watched Western fairy tales like Rupunzel.
-It could also be to emphasize their foreignness or to make them more of a wonder from a Japanese perspective (where blonde hair is rare).
-It could also be a way to visually make them more distinct from the rest of the cast. Nice or elaborate clothing, soft features, but also long blonde hair instead of dark hair like the commoners or lower-ranked nobles.
-It could be because yellow was a royal colour in Japan (like purple in Britain)
https://underthemoonlight.ca/2021/01/27/mikado-yellow-the-imperial-royal-colour-of-past-empires Same in some other East Asian cultures
https://theasiadialogue.com/2015/07...me-colour-at-the-top-of-the-social-hierarchy/ For some time during the Qing Dynasty, only the the highly ranked nobles and the monarchs could wear shades of yellow (with certain shades of yellow reserved for the highest ranks) and lower ranked nobles had to wear blue. In the earlier Ming Dynasty, red was the top colour amongst officials, but yellow, purple and reddish-black were only allowed to be worn by royals (and not by officials or commoners).
-For princesses/princes, it could signify youth, as many blondes (both in Europe and in Melanesia, whose blonde genes evolved independently of each other. Kinda ironic with Melanesia, since the etymology of Melanesia is from "melanin" and blonde hair is low in melanin (small amount of eumelanin and pheomelanin. If they had more pheomelanin they'd have redder hair, if they had more eumelanin they'd have browner/if lots of it black hair) become brunettes as they get into their teens or adulthood.
I doubt whether
princesses (need to read the title properly before typing) royalty in anime do have blonde hair that much though.
In Berserk, Charlotte, Julius and the King had brown hair, despite being European.
I don't know who the princess in LotGH is, but they seem to have many non-princess blondes too so it might have just been 50-50 on having a blonde princess.
Gintama's princess is non-blonde (Soyo), so is Yugioh's (Adena). King Furry in DBZ has blue-grey hair. Kingdom has a blonde princess, which is weird since it's Chinese, so you could be onto something. Black Butler has blonde ones too, but those are based on real people.
Vinland Saga has royalty based on real people, eg: Sweyne, Harald II (blonde in non-contemporary portraits, brunette in the series) and Cnut (who died at 45, ie quite young to be fully grey) is portrayed with light hair in contemporary art (
here/
here) and centuries later as
brunette (in 1802, way after his death) portraits and is blonde in the series)).
?
