naughty blue

[raisinbread]

How did the color blue get offcolor associations like blue laws, blue humor and the like? What’s the etymology for associating this color with the prurient?

[Snetho]

…don’t forget blue balls.

[everton]

Apparently “blue laws” were originally printed on blue paper (although we don’t have that expression in my country, so I can’t vouch for that). A Google search on “blue laws” etymology produces several references for it, and this one is a search within dictionary.com.

Blue humo(u)r will no doubt share its origin with blue movie. From this site:

*blue movie
an indecent or pornographic film - The word ‘blue’ became a euphemism for ‘indecent’ in the early nineteenth century. David Meeker of the British Film Institute explains: ‘Travellers returning from the Far East reported that Chinese brothels were painted blue so they could easily be identified.’ By 1864 the term was in common usage.

Cultural Note:
‘Other countries use different colours for steamy films,’ adds Meeker. 'In Spain and some South American countries they are GREEN because that is the colour of the censor’s certificate. In Italy they are RED because of the association with red-light districts. *

Not all uses of the word “blue” allude to prurience of course - blue riband, blue blood, blue-eyed boy etc.

[CalMeacham]

IIRC, Snopes and other debunkers have demolished the claim about blue laws being called that becaus they were printed on blue paper. The idea of blue Chinese brothels sounds pretty much like a UL to me.
Don’t forget “Blue noses”, too. The Monty Python movie And Now For Something Completely Different actually has a prude with a blue nose in one cartoon segment.

[yabob]

The Word Detective says, concerning “blue laws”:

The term “blue laws” seems to have been invented by Reverend Samuel Peters, a pro-British American clergyman whose “General History of Connecticut,” published in 1781, set out to paint the colonists as religious fanatics. Although popular legend maintains that the term “blue laws” arose because the laws themselves were printed on blue paper, Peters himself explained that by “blue” he meant “bloody,” i.e., enforced by whipping, maiming and death.

http://www.word-detective.com/051600.html#bluelaws

Quinion of World Wide Words agrees on that point, while being uncertain of the origin of “blue” for pornographic in general, though dating first use to 1820 in Scotland:

World Wide Words: Blue