Green’s Dictionary of Slang

(US) a newspaper.

1938		D. Runyon ‘The Big Umbrella’ in Runyon on Broadway (1954) 549: As if he just comes from a high-toned wedding such as you see in pictures in the Sunday blats.
1944		D. Runyon Runyon à la Carte 61: The blats keep printing his picture.
1960, 1975		Wentworth & Flexner DAS.
1989		J. Morton Lowspeak.