Green’s Dictionary of Slang

(US black) sheets and blankets.

1944		Cab Calloway New Hepsters Dict. in Calloway (1976) 254: dreamers (n.): bed covers, blankets.
1946		Mezzrow & Wolfe Really the Blues 219: I haven’t even seen my dreamers in a deuce of brights.
1960, 1975		Wentworth & Flexner DAS.