From Old French 'bleu,' Frankish *blāo, PIE *bʰlēw- (light-coloured) — a root so colour-ambiguous it also produced Latin 'flavus' (yellow).
Of the colour intermediate between green and violet in the spectrum, resembling a clear sky or deep sea.
From Middle English 'bleu,' borrowed from Old French 'bleu,' which itself came from Frankish *blāo, from Proto-Germanic *blēwaz meaning 'blue, dark, livid.' The native Old English word was 'blǣwen' (blue), but it was largely displaced by the French borrowing after the Norman Conquest. The ultimate PIE root is *bʰlēw-, meaning 'light-coloured, blue, blond, yellow,' reflecting an ancient
Many ancient languages, including Homeric Greek and Biblical Hebrew, had no dedicated word for blue — Homer famously described the sea as 'wine-dark' (oinops), and some scholars have argued that the cognitive perception of blue as a distinct category may require a linguistic label for it.