Green’s Dictionary of Slang

(US campus) a brooding, gloomy adolescent who wears dark clothes and listens to gloomy alternative music.

1990		Eble Campus Sl. Apr.
1996		Eble Sl. and Sociability 37: Two kinds of shortening reduce words to letters. The dominant type in college slang is initialism, which names the individual letters: [...] PIB (people in black) ‘brooding, gloomy teenager who wears dark clothes and listens to alternative music about death’.
1997–2002		Hope College ‘Dict. of New Terms’ 🌐 PIB n. Shortened form of Person In Black. Someone who wears either all black or black clothing all the time. Used in the larger U.S. cities. Oftentimes, a PIB is one who is a ‘deep,’ artsy intellectual or a very chic, cosmopolitan type.