An Etymological Dictionary of Astronomy and Astrophysics

The hue of that portion of the visible spectrum lying between green and indigo, evoked in the human observer by radiant energy with wavelengths of approximately 420 to 490 nanometers.

Etymology (EN): From O.Fr. bleu, P.Gmc. *blæwaz, from PIE base *bhle-was “light-colored, blue, blond, yellow.”

Etymology (PE): Âbi “color of water,” from âb “water,” Mid.Pers. âb, O.Pers./Av. âp-, Skt. âp-, PIE *âp-; → Aquarius.