What word does your mother always say?
c.1300, bleu, blwe, etc., from Old French blo "pale, pallid, wan, light-colored; blond; discolored; blue, blue-gray," from Frankish *blao or some other Germanic source, from Proto-Germanic *blæwaz (cf. Old English blaw, Old Saxon and Old High German blao, Danish blaa, Swedish blå, Old Frisian blau, Middle Dutch bla, Dutch blauw, German blau "blue"), from PIE *bhle-was "light-colored, blue, blond, yellow," from PIE root bhel- (1) "to shine, flash" (see bleach (v.)).
The same PIE root yielded Latin flavus "yellow," Old Spanish blavo "yellowish-gray," Greek phalos "white," Welsh blawr "gray," Old Norse bla "livid" (the meaning in black and blue), showing the usual slippery definition of color words in Indo-European The present spelling is since 16c., from French influence (Modern French bleu).
The exact color to which the Gmc. term applies varies in the older dialects; M.H.G. bla is also 'yellow,' whereas the Scandinavian words may refer esp. to a deep, swarthy black, e.g. O.N. blamaðr, N.Icel. blamaður 'Negro' [Buck]The color of constancy since Chaucer at least, but apparently for no deeper reason than the rhyme in true blue (c.1500). From early times blue was the distinctive color of the dress of servants, which may be the reason police uniforms are blue, a tradition Farmer dates to Elizabethan times. For blue ribbon see cordon bleu under cordon. Blue whale attested from 1851, so called for its color. The flower name blue bell is recorded by 1570s. Blue streak, of something resembling a blt of lightning (for quickness, intensity, etc.) is from 1830, U.S. Western slang.
Few words enter more largely into the composition of slang, and colloquialisms bordering on slang, than does the word BLUE. Expressive alike of the utmost contempt, as of all that men hold dearest and love best, its manifold combinations, in ever varying shades of meaning, greet the philologist at every turn. [John S. Farmer, "Slang and Its Analogues Past and Present," 1890, p.252]
"lewd, indecent" recorded from 1840 (in form blueness, in an essay of Carlyle's); the sense connection is unclear, and is opposite to that in blue laws (q.v.). John Mactaggart's "Scottish Gallovidian Encyclopedia" (1824) containing odd words he had learned while growing up in Galloway and elsewhere in Scotland, has an entry for Thread o'Blue, "any little smutty touch in song-singing, chatting, or piece of writing." Farmer ["Slang and Its Analogues Past and Present," 1890] offers the theory that this meaning derives from the blue dress uniforms issued to harlots in houses of correction, but he writes that the earlier slang authority John Camden Hotten "suggests it as coming from the French Bibliothèque Bleu, a series of books of very questionable character," and adds, from Hotten, that, "Books or conversation of an entirely opposite nature are said to be Brown or Quakerish, i.e., serious, grave, decent."
"to make blue," c.1600, from blue (1).
generally associated with purple (Ex. 25:4; 26:1, 31, 36, etc.). It is supposed to have been obtained from a shellfish of the Mediterranean, the Helix ianthina of Linnaeus. The robe of the high priest's ephod was to be all of this colour (Ex. 28:31), also the loops of the curtains (26:4) and the ribbon of the breastplate (28:28). Blue cloths were also made for various sacred purposes (Num. 4:6, 7, 9, 11, 12). (See COLOUR.)