Several languages, including Japanese, and Lakota Sioux, use the same word to describe blue and green. In Russian, Spanish and some other languages, there is no single word for blue, but rather different words for light blue (голубой, goluboj Celeste) and dark blue (синий, sinij Azul). In heraldry, the word azure is used for blue. The modern English word blue comes from Middle English bleu or blewe, from the Old French bleu, a word of Germanic origin, related to the Old High German word blao (meaning shimmering, lustrous).

The same surveys also showed that blue was the colour most associated with the masculine, just ahead of black, and was also the colour most associated with intelligence, knowledge, calm, and concentration. In US and European public opinion polls it is the most popular colour, chosen by almost half of both men and women as their favourite colour. Surveys in the US and Europe show that blue is the colour most commonly associated with harmony, faithfulness, confidence, distance, infinity, the imagination, cold, and occasionally with sadness. Distant objects appear more blue because of another optical effect called aerial perspective. An optical effect called Tyndall effect explains blue eyes. The clear daytime sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. Most blues contain a slight mixture of other colours azure contains some green, while ultramarine contains some violet. The eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres. It lies between violet and cyan on the spectrum of visible light.

Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model.

Blue horizon