There is no consensus as to how the term "blue film" came to be associated with obscene or pornographic movies.

It is interesting to note that the first record of the color blue to refer to obscene or risque material appeared in 1824 in Scotland. It was an entry in The Scottish Gallovidian Encyclopedia by John MacTaggart referring to a " Thread o’ Blue" as "any little smutty touch in song-singing, chatting, or piece of writing".

However, there does not seem to be any link between that record and how pornographic movies came to be called blue films.

One theory is that the term came to be in use from the fact that in the 1980's, pornographic content in video cassettes was sealed using a blue tape.

Another theory suggests that it started with the blue bars across the faces of the actors or actresses to obscure their identities.

And since striptease acts might have used blue spotlights, this might also have been the origin of the blue in blue film.

Yet another theory attributes it to the bluish color tint evident in the earliest pornographic films. Those films were in black and white, and the bluish tint appears when quick and cheap ways were used to develop the film.

I did a quick check and found that the color blue is often associated with depth and stability. It symbolizes trust, loyalty, wisdom, confidence, intelligence, faith, truth and heaven.

It seems that the color blue is associated with and symbolizes great stuff.

I am wondering if I start watching blue films some of those great stuff will rub off. You will never know until you try, right? ;)

References:

1. World Wide Words

2. Yahoo! Answers

3. Urban Dictionary

4. Associated Content

5. Color Wheel Pro