Isn't it weird for little countryside girl to dye her hair? She can't be blond if she is Japanese.
Manga world. Some of these have natural born pink haired girls, which don't exist on Earth, let alone in Japan. Someone with western blood who was born in Japan is still Japanese, too, anyway. Also, you can apply for Japanese citizenship which would technically make you legally Japanese-ish (everyone and their mother would still probably treat you like some random gaijin). Variety of avenues to go with here for an explanation on why some girl from Hiroshima has blonde hair.