blue sky, n.³ — Green’s Dictionary of Slang

blue sky n.³

also sky

[? blue sky v.; i.e. the dreaminess produced by the drug]

(US drugs) heroin.

1986		C. Stroud Close Pursuit (1988) 48: His character was ‘Ramon,’ ex-psychiatric case summa cum yo-yo from Bellevue with a minor in blue sky and toot. [Ibid.] 305: He gives me free methadone so I feel better. I don’t need sky if I have my doctor.
1990		in Paul Dickson Slang!