Green’s Dictionary of Slang

black stuff n.¹

1. opium.

1935		A.J. Pollock Und. Speaks.
1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 118/2: black stuff. Opium, as contrasted to white stuff (morphine). Not used to refer to opium after it has been prepared or cooked for smoking, when it becomes a pill or yen-pok.
1949		Monteleone Criminal Sl. (rev. edn).
1950		S.J. Baker in Sun. Herald (Sydney) 8 June 9/4: Among American borrowings recorded in Detective Doyle's list are: ‘Blow,’ to depart, go away; ‘boob’ and ‘can,’ gaol; ‘black stuff,’ opium.
1959		J.E. Schmidt Narcotics Lingo and Lore.
1969		R.R. Lingeman Drugs from A to Z (1970).
1971		E.E. Landy Underground Dict. (1972).
1981		D.E. Miller Bk of Jargon 324: Opium. Also black stuff. This oldest of the opiates can be eaten or smoked, but not injected.
2001		ONDCP Street Terms 3: Black stuff — [...] opium.

2. stout, esp. Guinness.

1965		L. Dunne Goodbye to The Hill (1966) 122: She didn’t eat much but she was putting the black stuff away as quick as I was myself.
1996		Irish Times 5 Jan. 🌐 It [Iceland’s first Ir. pub] apparently sold 100,000 pints of Guinness in its first two weeks. A Bjorn again market for the black stuff .
2000		J. Hawes Dead Long Enough 278: I get shitfaced about once every five days on more than just the black stuff.

3. heroin.

2001

ONDCP Street Terms 3: Black stuff — Heroin.

In phrases

shovelling the black stuff

(US drugs) addicted to opium.

1959

J.E. Schmidt Narcotics Lingo and Lore.