Learn English – What’s the meaning of “hit somebody like a bucket of cold water”

'You've got two chances--slim and none...and Slim left town'.

Adding to StoneyB's answer, in the first half of the 20th century "blue bag" was also a trade name for a well-known brand of starch used when washing white clothes. Various blue-colored compounds were mixed with the starch in order to to wash clothes "whiter than white".

There was a well known intellectual property dispute between two manufactures of this type of product. The legal action arising from this is still cited in English law disputes. (Ironically, the two companies later merged with each other!)

http://www.oldandinteresting.com/laundry-blue.aspx

The connection with "washing dirty jokes clean" is obvious.

As an amusing irrelevance, in the area surrounding Reckitt's factory in Hull which produced "Reckitt's Blue", the colours used to identify the main UK political parties differed from the nationwide conventions. The owner of Reckitt's was a committed supporter of the Liberal party, and the local colours if the Liberals and Tories were blue and yellow respectively - the opposite of the rest of the UK. This became even more confusing after Reckitt's merged with Colman's, whose best known product was mustard!