Black radishes are a type of radish known for their strong flavor. They come from Asia but have been used in the Middle-East and Europe for centuries. As the name suggests, they are black on the outside (but white on the inside). While they can be (and are) eaten, they are most often talked about as a type of natural remedy.
Black radishes have a lot of vitamin C and some B vitamins, making it good healthy eating for the peasants during the winter. Since is has a lot of fiber it also helps fight against parasites. It also contains antioxidants. As a natural remedy it can treat sore throat and cough, help relieve trapped gas, act as a diuretic, and 'stimulate the bile', being an old-timey way of saying 'help the liver'.
Black radishes are one of the spiciest types of radish, and although it is still used as a garnish in Europe it's not eaten in many places. Asian varieties usually have longer roots than the European radishes. When you eat a black radish, they are usually salted and rinsed first, to tame the taste a little.
A black radish is sometimes called a winter radish, because they are harvested in the winter.