Green’s Dictionary of Slang

1. a police car, thus v. blue light, to indicate that a vehicle should stop for questioning etc.

1939		Howsley Argot: Dict. of Und. Sl. 8: blue light – a narcotic officer’s car.
1988		Ice-T ‘Drama’ 🎵 fuckin’ blue lights – read ’em their rights! drama!
2005		J. Buskey Tinged Valor 48: I waited for police backup, blue lighted the vehicle, and took all occupants into custody.
2013		G. Crowe ‘Of Being Darker Than Light’ in ThugLit Feb. [ebook] Before he was blue-lighted, Rex sped by the road lines.

2. (US) a police officer.

2002

Ryan Adams ‘Rosalie’ 🎵 The blue lights had his number.