Wiktionary
n. An escharotic based on bloodroot extract which is used as an alternative cancer treatment.
Wikipedia
Black salve, also known by the brand name Cansema, is a dangerous and controversial alternative cancer treatment. The product is commonly classified as an escharotic—that is, a topical paste which burns and destroys skin tissue and leaves behind a thick, black scar called an eschar. Escharotics were widely used to treat skin lesions in the early 1900s, but have since been replaced by safer and more effective treatments. Escharotics, such as black salves, are currently advertised by some alternative medicine marketers as treatments for skin cancer, often with unsubstantiated testimonials and unproven claims of effectiveness.
The U.S. Food and Drug Administration has listed Cansema as a "fake cancer cure" and warns consumers to avoid it.
Usage examples of "black salve".
As it went from hand to hand he took a small round box from his cloak, dug out a nailful of black salve on his index finger, and rubbed it into the round hole in his shirt.