blue from Middle English:
The English blue and French bleu are ultimately the same word, which goes back to ancient Germanic and is related to the blae- in blaeberry (Middle English), a Scottish and northern English name for the bilberry (late 16th century). Blue occurs in a number of phrases, in particular those relating either to depression and melancholy or to the blue of the sky, as in out of the blue, ‘as a total surprise’. See also bolt. Something occurring once in a blue moon is something very rare. A blue moon sounds fanciful but it is a phenomenon that does occur occasionally, due to large amounts of dust or smoke in the atmosphere. A particularly Australian use of blue is as a humorous nickname for a red-haired person. This is first recorded in 1932, although bluey is earlier, from 1906.
Depression or melancholy have always been around, but no one called these feelings the blues until the mid 18th century, although people have been feeling blue since as early as the 1580s. The blues was a contraction of blue devils, which were originally baleful demons punishing sinners. In the 18th century people fancifully imagined them to be behind depression, and later also to be the apparitions seen by alcoholics in delirium tremens. The first printed record of the name of the melancholic music style is in the ‘Memphis Blues’ of 1912, by the American musician W. C. Handy, who later set up his own music-publishing house and transcribed many traditional blues. Its later development, rhythm and blues, appeared in the 1930s.
Obscene or smutty material has been known as blue since the mid 19th century. The link may be the blue gowns that prostitutes used to wear in prison, or the blue pencil traditionally used by censors.
Blue-chip shares are considered to be a reliable investment, though less secure than gilt-edged stock (used since the later 19th century for government stock, and earlier to suggest excellent quality). Blue chips are high-value counters used in the game of poker. In America a blue-collar worker (mid 20th century) is someone who works in a manual trade, especially in industry, as opposed to a white-collar worker (early 20th century) in the cleaner environment of an office. A blueprint (late 19th century) gets its name from a process in which prints were composed of white lines on a blue ground or of blue lines on a white ground. See also murder