Green’s Dictionary of Slang

1. (US) a pious, sanctimonious individual; also as adj.

1821–6		‘Bill Truck’ Man o’ War’s Man (1843) 109: Them there mulatto chaps [...] take on as smartly [...] as a blue-light.
1850		(con. 1843) Melville White-Jacket (1990) 49: The holders of our frigate, the troglodytes, who lived down in the tarry cellars and caves below the berth-deck, were, nearly all of them, men of gloomy dispositions, taking sour views of things; one of them was a blue-light Calvinist.
1851		Manchester Spy (NH) 10 May n.p.: Our wise grannies [...] can beat the blue lights, and throw them into the hade for petty tyranny and nonsense.
1877		H. Hartigan Stray Leaves (1st ser.) 98: [T]he reception the syce would get from old blue-light (the serjeant-major’s nickname) when he went for his rupee in the morning.
1882		Godfrey Life of George Godfrey 29: I had to put up with being called a Methodist blue-light.

2. (US campus) a student who informs on other students to the authorities.

1856		B.H. Hall College Words (rev. edn) 30: blue-light [...] A blue-light is occasionally found watching the door of a room where a party of jolly ones are roasting a turkey (which in justice belongs to the nearest farm-house), that he may go to the Faculty with the story, and tell them who the boys are.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 23: blue-light, n. A student who seeks to ingratiate himself with the faculty by informing.