Although the origin of the phrase "blue movie," to refer to a pornographic film, is unclear, it may be derived from the French series of books known as the "Bibliotheque Bleue," or "blue library." Produced between the 17th and mid-19th century, these books reflected the popular culture of their times, covering topics such as burlesque and chivalrous romance. While some of these titles covered less risque or even more virtuous topics, such as how-to and religious instruction guides, their target audience of the peasant class and their low production quality led to their reputation for "questionable character."
Another theory of the phrase's origin, albeit somewhat less credible, is that "blue" came to be associated with loose morals when it was designated as a uniform color for harlots in houses of correction. An early use of the term "blue" in reference to lewdness or indecency can be found in John Mactaggart's 1824 "Scottish Gallovidian Encyclopedia," where the phrase "Thread o'Blue" is defined as "any little smutty touch in song-singing, chatting or piece of writing."
A popular theory holds that the use of the word "blue" to refer to smut comes from the 17th century "blue laws" of the New Haven colony. Among other things, these laws were established to uphold moral values by penalizing people for indecent behavior.