What is the meaning behind “blue movie”

The origin of “blue” as a descriptor for something sexual or lewd is not known in a definitive way. But I have a theory about why “blue movie” came to be used as a label for pornographic movie.

The clue is found in a synonym for blue movies from the 1930s. A “smoker.”

Pornographic movies were first shown in private clubs, bordellos, the back rooms of saloons, and other places where an audience of men came to see them. And in the era we’re exploring—let’s say 1900 to 1960, most men smoked tobacco. Cigars or cigarettes.

In a room with a low ceiling and a movie projector at one end, heavy tobacco smoke looks blue in color when the neutral light of a black-and-white movie shines through it. The particles in the air shift the wavelength of light toward the blue end of the spectrum, just as dust particles in the air make the sky look blue due to scattering.

Contrary to many of the answers which sound pretty good my understanding is based on conversations and presentations with and by people who worked in Hollywood in the early days of the industry.

Film equipment (specifically the cameras) in the 1920s and 30′s was owned by the studios. There weren’t any rental companies. Consequently, if you wanted to make an ‘off the books’ film you had to use their equipment. The buildings in which films where shot (called sound stages once ‘talkies’ came out) had 2 sets of lights at the doors. One light was Red the other was Blue. If the Red light was on it meant a camera was rolling, basically a do not disturb sign. On the other hand if the Blue light was on then it was OK to enter. The film is called blue movie

Since unsanctioned films, mostly what would be called pornography (although from what I have seen much of it would barely make an R rating by today’s standards) were shot after hours. So it was understood (wink, wink, nudge, nudge) that when the Blue light was on after hours it really meant that a camera was rolling the same meaning as the Red light during the day. Hence film produced this way were referred to as Blue films.

Initially in the 19th century, the reels or films which were used for normal movies were white in color and they were costly to use. B-grade or adult movies were basically taken in a low budget thereby they were not able to buy white reels or films, they bought blue video reels which are cheaper. This is the reason adult movies are also known as blue films.

The term “blue” to mean lewd or obscene dates to at least the early 1800s. Songs or stories with suggestive or bawdy content such as sexual double-entendres were said to have a “thread of blue” in them. (Gallovidian Encyclopedia, 1824)

When movies came along around 1900, the term “blue video” was applied to the new medium, which was referred to as “film” in reference to the celluloid film (thin layer) upon which the images were imprinted. Nobody knows why “blue” came to be associated with lewd content, as the origin of the term is not recorded.