Black teas undergo a more rigorous oxidation. Also called red tea in China due to the red colour of their infusions, black teas are completely oxidized.
Legend has it that in the 17th century a shipment of green tea from China arrived in London following a particularly long journey. During the trip, mould had grown in the crates and the tea inside had turned black. Little did they know that the British would acquire a taste of this tea and immediately order more!