Q From Miriam Shlesinger, Israel: I wonder whether you could tell me the origin of blue in the sense of pornographic.
A I wish I could.
What we do know is that the word began to be applied to matters obscene in the 1820s. (There’s an entry for it in The Scottish Gallovidian Encyclopedia by John MacTaggart of 1824: “Thread o’ Blue, any little smutty touch in song-singing, chatting, or piece of writing”).
So it long pre-dates, for example, any reference to the musical genre called the blues, or to the use of editorial blue pencils to delete anything suggestive, which seems to be a late nineteenth-century joke based on the pornography, by then well established.
There are several theories about where it came from. It has been said that prostitutes who were imprisoned in a house of correction were forced to wear blue gowns, and as a result bluegown was used for a prostitute, with blue being derived from it by abbreviation (this has nothing to do with another sense of bluegown, which you will find in the piece about gaberlunzie, nor indeed to a bluestocking).
Another theory is that it derives from the series of French books entitled Bibliothèque bleue, which John Farmer in the first volume of his Slang and its Analogues of 1891 attribute to the earlier slang compiler John Camden Hotten. Farmer described them as being “of very questionable character” But La bibliotèque bleue was actually a tradition of inexpensive publications originally bound in blue paper covers, ranging from practical and pious works to entertainments similar to the British penny dreadfuls. Farmer might have been familiar only with the last type.
The American expression blue law exists for a puritanical law that prohibits certain activities, especially on the Sabbath. In fact, it precedes the first use of blue in the sense of a thing that offends against morals by about 40 years. It seems to have been popularised by the Reverend Samuel Peters, in a work published in London in 1781, The General History of Connecticut. In it, he roundly criticises the ultra-strict puritan laws in that state. He called them blue laws because they were “bloody laws; for they were all sanctified with whipping, cutting off the ears, burning the tongue, and death.” Why this makes the laws blue rather than red isn’t clear.
It is sometimes said that the original blue laws of Connecticut were so called because they were printed on blue paper or between blue covers, but as no example of the term has been found that precedes Peters’ book, it is thought he invented the term, and that this idea is a later rationalisation.
No direct link has been established between this usage and the first example (from Scotland, remember) of the use of blue in the sense of something smutty, but it is not wholly improbable.