This discussion on a linguistic forum states that Belarusian ‘blakitny’ (not to be mixed with dark blue) exists in Ukrainian (блакитний), Polish (błękitny), Czech (blankytný). The term is archaic in other Slavic languages? It’s not in Belarusian and Ukrainian.
It does not look as the term is Slavic in origin. It is either Latin or old French originally.