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 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.
Many Indo-European languages seem to have had a word to describe the color of the sea, encompasing blue and green and gray; e.g. Irish glass (see Chloe); Old English hæwen "blue, gray," related to har (see hoar); Serbo-Croatian sinji "gray-blue, sea-green;" Lithuanian šyvas, Russian sivyj "gray."