But before the Sellers movie there was a sub-B British flick from 1933 that this site cites as THE worst movie ever made, beating even Plan 9 From Outer Space for that ambiguous honor:

http://www.kjenkins49.fsnet.co.uk/shotinth.htm

The movie was adapted from a novel and stage play of the same name by Gerard Fairlie (1899-1983), a show I remember seeing staged as a boy at a summer stock theatre in Middlesex, NJ, called Foothills Playhouse, when my parents were active with the company there. Except Mr. Fairlie's original title was without the "A"...Shot in the Dark. He also wrote in the crime genre, so there's no phrase-spawning metaphors here. The novel was published in 1932.

[Edit] Found this on a site called The Phrase Finder http://phrases.shu.ac.uk/meanings/, which is now, evidently, a subscription service. But you can still get into the Meanings and Origins of Phrases link and search. They don't supply any dates or coining citations, though. And none of the top dictionaries or dictionaries of phraseology ( that I searched) carry this phrase??? Too obvious?

Shot in the dark- A

Meaning - A hopeful attempt at something.
Origin - Like a hopeful attempt to shoot at an enemy that you can't see.