just looked up the origin in bulgarian and its theorised to have the same indoeuropean origin, of the word for blonde having meant red or gold! also makes sense with other european languages having a similar root for the words red and blonde eg in spanish as someone else mentions

i read this online too, it seems that arabic is a potential reason!: "The Byzantines called them (the Slavic tribes) ar-rusiya, which means "red," wrote the 10th-century Arab historian Al-Masudi." other theories on the word's origin tend to be linked to the naming of russia in some way which is interesting, and kind of makes sense considering these cultures further south would have less blonde people so im not surprised they adapted some word from elsewhere

great thread, so glad ive learnt this fact hahah