DO NOT GOOGLE BLACK SALVE! The injuries that can occur to people using this ‘treatment’ are horrific, so you have been warned. Black salve is made up of zinc chloride and herbs (most commonly chapparral) and bloodroot. Bloodroot extract is known as sanguinarine and is a quaternary alkaloid - this means it attacks living tissue and is classified as an harmful "escharotic".
Black salve is marketed as an alternative cancer treatment. Proponents of the treatment claim that the burning of the tissue is part of the treatment attacking the cancer and aiding recovery - in reality this is causing serious permanent harm. These salves were first labelled a quack treatment in 1955 but their use still continues to this day.
Black salve is banned by the FDA and Australian regulators. It is only available online in the UK, and selling it as a cancer cure would be covered by 1939 Cancer Act which forbids selling any products as a cancer cure. These salves are used to treat other skin conditions as well. I have been unable to find if there is any UK/EU legislation that limits their sale in the UK.