During processing, black teas go through an oxidisation phase. The freshly plucked leaves are crushed, releasing enzymes which turn the leaf black whilst also deepening and intensifying its flavour. The longer a tea is oxidised, the darker and stronger it becomes, and the process is halted by firing (heating) the leaves. The inherent skills of plucking, processing and sorting the finished leaves determines the quality of the tea.
See information on leaf types