Green’s Dictionary of Slang

blue devil n.2

[packaging]

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

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