The common causes of black patches on the back of the feet include: ultraviolet radiation, melanoma, trauma and so on. 1. Ultraviolet radiation: if the instep is irradiated by ultraviolet rays for a long time, the skin will be easily damaged to a certain extent, and the melanocyte function will be active, which in turn will result in a large number of melanin granules deposited in the damaged skin, producing the phenomenon of black patches on the instep. It is recommended to avoid strong light irradiation and do a good job of sun protection. 2. Melanoma: It is a kind of skin malignant tumor originated from melanocytes. In the early stage of the lesion, the original mole area will increase rapidly and bulge, which will result in black spots and can occur in any part of the body. With the progress of the lesion, there will be metastasis, ulcers and other manifestations. 3. Trauma: If the back of the foot is squeezed, stepped on or hit, resulting in trauma, which leads to blood stagnation in the skin, it can be manifested as a black plaque. After the wound heals, it will also cause hyperpigmentation, which will also manifest as black patches. In conclusion, if you have black patches on the back of your feet, it is recommended that you go to a regular hospital for a clear diagnosis to avoid adverse consequences.