"Black beauty" was a drug street-name used in the '60s and '70s to refer to a pill of pharmaceutical amphetamine (aka speed). Strictly speaking, a black beauty was a tablet of Biphetamine, manufactured by Strasenburgh Labs, then Pennwalt Corporation, and finally Fisons Corporation, before finally being pulled from the market in 1998. Chemically speaking, it was composed of an even, racemic mixture of dextro- and levo- amphetamine, very similar to today's Adderall. Contrary to what other definitions might say, both the d- and l- isomers of amphetamine are active and "fun." In fact, while d-amp is technically the stronger isomer, the racemic d/l mixtures are actually more stimulating, more abusable, and create more of a drug "high."
The term "black beauty" was also extended to include Biphetamine-T, which was a ridiculously abusable mixture of the aforementioned amphetamines, plus methaqualone (aka Quaalude). Both Biphetamine mixtures were available in 12.5mg and 20mg tablets.
Black Beauties were popular in the 60s and 70s, especially following post-war eras, when it stopped being shipped to our soldiers overseas and suddenly built up as a surplus on the home front. This amphetamine surplus eventually trickled out onto the streets where it was consumed recreationally. Amphetamines were not made illegal to buy without a prescription until 1965, so amphetamine use at the time was widely considered a cheap, legal alternative to cocaine, and its abuse wasn't considered taboo. It wasn't uncommon for housewives of the '50s and '60s to secretly pop speed pills and proceed to go on marathon 8-hour cleaning sessions. Amphetamine use continued rather uninhibited until that asshole Nixon came into office and passed the Controlled Substances Act in 1970 (aka the law under which all street drugs are now illegal) and "The War on Drugs" was born. Pharmaceutical amphetamine abuse dwindled after that, and all the speed pills of the '60s and '70s began to disappear off the market. By the late '90s, most pills were just a distant memory. Of course, the past decade has brought a sudden rash of ADD/ADHD cases (or diagnoses rather), so now we're in the middle of a new speed "epidemic." Kids everywhere are once again abusing uppers (Ritalin/Adderall/Dexedrine) just like your parents abused black beauties in the early '70s.