I haven't googled the term, (actually I use Startpage, Google is getting a little 1984ish), but I know the answer. I read it once in a NY Times Magazine article back in the late sixties or early seventies.
The expression "blue movie" comes from the term "getting blue", a term I used to hear my parents and aunts and uncles use, but which nobody from my generation onward uses. It means to start telling jokes with sexual content, something which sometimes happened later at night in an age when people were not so careful about the number of drinks consumed at parties as they are now.
And the reason it was called "getting blue" was that movies back in the 1940's, (and possibly earlier), had censors. Later, TV had them too. The censors crossed out the objectionable, (sexual) parts of the script with a blue pencil. So any part of a movie or TV script that was over the line sexually, (and that was a very prudish age, by modern standards), was considered "getting blue", that is, in danger of getting censored.
When porn flicks came in, obviously the whole movie would have been censored, hence "blue movie".