blue

   1. American

   a policeman or prison warder

   From the normal colour of the uniform. Also as a bluebottle, bluebird, blue-belly, blue jeans, blue suit, or blue-and-white:

    Okay, [the elevator] was on the sixth floor when the first blues got to the Kipper townhouse. (Sanders, 1980)

    We blue suits liked the mouse. (King, 1996 — they were prison warders)

   In Britain a man in blue or a bluecoat is a policeman, working perhaps out of a blue lamp, a police station, named after the standard exterior lit sign.

   For the Nazis, the blue police were to enforce their rule in occupied territories:

    ... speak to the SS men, to the Ukrainian auxiliary, to the Blue Police and to the OD details. (Keneally, 1982, writing of Nazioccupied Poland; German control rested with the army and these four organizations in descending order of importance, the OD being Jews placed in authority over other Jews)

   2. erotic

   Probably from the French bibliothèque bleue, a collection of seamy works of literature, rather from the colour of the brimstone which awaits evil-doers:

    She starred in dozens of blue movies before coming above ground. (Deighton, 1972)