Green’s Dictionary of Slang

(drugs) a depressant, esp. Phenobarbitone (luminal), amobarbital.

1967		J.B. Williams Narcotics and Hallucinogens.
1971		Chronicle-Telegram (Elyria, OH) 1 May 7: Among the common commercial names for barbiturates are [...] Amutal or ‘blue heavens’ or ‘blue devils.’.
1977		S.N. Pradhan Drug Abuse.
2001		ONDCP Street Terms 3: Blue devil — Depressants.