When you said B movie I mistook it for "blue film" which is slang for porn in India. You can imagine my shock for a second. I recovered.

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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. {Read on}

The color "blue" is symbolic of many things, yet the most curious may be matters of obscenity and puritanical decree. The Word Detective notes the phrase "blue laws" dates back to 1781 when the Reverend Samuel Peters published his history of Connecticut. He painted the strict laws of the puritan colonists as "blue laws," meaning "bloody laws" or laws that were enforced by brutality. "Blue laws" became shorthand for any strict, old-fashioned laws, such as laws forbidding liquor sales on Sundays. {Read on}