Blue humor, blue movies, blue talk—what’s so obscene about the color blue?
Nobody really knows, as it turns out. The origin of blue in the sense of lewd, coarse, or pornographic has been tough to pin down: Etymologists have put forward a bunch of theories but haven’t found anything conclusive.
The Oxford English Dictionary’s earliest example of this usage dates back to 1818, in John Mitford’s The Adventures of Johnny Newcome in the Navy, in which Mitford (under pseudonym Alfred Burton) wrote, “Blush, Pluto! Blush as brimstone blue! This bluer Town can boast like you A ‘facilis descensus’ too.” I can’t find evidence that this blue-as-brimstone metaphor for sin really caught on, though. John Mactaggart’s Scottish Gallovidian Encyclopedia, from 1824, lists “Thread o’Blue” to mean “any little smutty touch in song-singing, chatting, or piece of writing,” which sounds more like the blue we’re after, but the encyclopedia doesn’t give any hints about its origins. In Slang and Its Analogues Past and Present (1890), John Stephen Farmer and William Ernest Henley proposed that blue might refer to the blue gown worn by a convicted prostitute in a house of correction, although that usage dates from the 16th century and doesn’t seem to have endured into the early 1800s. A related source for the crass blue is the color’s association in Britain with uniforms worn by servants and licensed beggars, who were not necessarily smutty but were certainly considered coarse and unrefined, paralleling the evolution of the term blue collar, which popped up in North America in the 20th century.
Slang authority John Camden Hotten, in his 1859 publication, A Dictionary of Modern Slang, Cant, and Vulgar Words, suggested that the base or indecent connotation of blue had its origins in the French Bibliothèque bleue, popular literature published between the early 17th and mid-18th centuries on low-quality paper with a blue cover and read by the lower classes. The OED disputes this conjecture, “since such material appears in general to have been highly moral in tone”—unlike, say, the blue books that emerged in Storyville, New Orleans’ red-light district, which were directories of the area’s prostitution services.
Whether swearing a blue streak has different origins compared with blue talk or blue language (not to be confused with Léon Bollack’s constructed language) is also up for debate. Blue streak may originally have simply meant fast or vivid, like a flash of lightning: an issue of the Kentuckian from 1830 featured the sentence, “To pass … with such rapidity as not even to leave a ‘blue streak’ behind him.” Swearing had wormed its way into the expression by 1847, as in “a ‘blue streak’ of oaths,” possibly independently of other sweary instances of blue.
Blue laws, which began in 1755 as puritanical restrictions on the activities of New England residents on Sundays, are unconnected with the obscene sense of blue. Contrary to popular belief, blue laws were never printed on blue paper and so the origin of their name also remains a mystery.
The term off-color evolved separately, appearing in the mid-19th century. According to the OED, it was first used to describe diamonds of lower clarity and thus took on the connotation of impure. I can’t find anything to suggest that off-color = blue.
While we’re at it, I should also mention that sacré bleu (or sacrebleu) isn’t an oath French speakers use (anymore). And even this interjection has a murky etymology. Depending on whom you ask, bleu might refer to the Virgin Mary, often depicted wearing a blue dress or sash, or it might be a mincing of dieu (God). I’m inclined to believe the latter, because other dieu → bleu minced oaths—including corbleu (for corps de dieu, or God’s body) and morbleu (for mort de dieu, or God’s death)—can be found in medieval French records.
The ribald blue is a bit of idiomatic language with a squishy, sordid history that I can’t offer you in a neat package—and is a great example of how equivocal etymology can be. If you’ve come across other theories about its origins, let us know in the comments.