Leaves are green because they contain a pigment called chlorophyll, which absorbs all wavelengths of light except for green, which is reflected. If there is no chlorophyll present in a leaf, then there will be no green color reflected, and the leaf will appear black.
This can happen when a leaf is injured or diseased, or when it is not getting enough sunlight. In some cases, a leaf may also turn black if it is exposed to high levels of pollution.