Green’s Dictionary of Slang

minstrel n.

also black and white minstrel, nigger minstrel
[the drug’s black and white capsules; thus a pun on the popular Black and White Minstrel Show (from 1960)]

(drugs) Durophet.

[UK]Glatt et al. Drug Scene in Grt Britain 115: Minstrel (black and white) – Durophet.
[US]R.R. Lingeman Drugs from A to Z (1970) 47: black and tan, black and white, black and white minstrel (from the colour) durophet capsules, 12.5 milligrams. [Ibid.] 191: nigger minstrel [...] durophet capusles.
[US]E.E. Landy Underground Dict. (1972) 34: black and white minstrel. [...] 12.5 mg. capsule of an amphetamine and a sedative.
[UK]S. McConville ‘Prison Language’ in Michaels & Ricks (1980) 526: Amphetamine-barbiturate mixtures seem to have spawned a particularly vivid range of nicknames and images, often arising from the appearance or color of capsules in which they are taken. These include [...] black and white minstrel, [...] nigger minstrel, [etc.].
[UK]J. Morton Lowspeak 104: Nigger minstrel – Durophet capsule.