The darkening of the joints on the back of the hand is considered to be related to the action of external forces, post-inflammatory hyperpigmentation, frostbite, necrosis and other factors. 1. The role of external forces: local external forces, such as contusions leading to subcutaneous bleeding, the local color may be blackened. 2. Post-inflammatory hyperpigmentation: the local area is subjected to long-term extrusion, friction or skin injury, and there is local inflammation in the process of recovery, and post-inflammatory hyperpigmentation, resulting in local color blackening. Rheumatism, rheumatoid arthritis, resulting in blood circulation disorders, the back of the hand at the joints of the color may be black. 3. Frostbite: in a cold environment, do not pay attention to cold and warmth, resulting in contraction of small arteries in the limbs, local vasospasm, poor blood circulation induced frostbite, resulting in localized skin black and purple. 4. Local tissue necrosis: the presence of severe local trauma, infection, resulting in some tissue necrosis, the skin color may be black. In summary, the back of the hand joints black color of the reasons are more, it is recommended to pay strict attention to sunscreen, if necessary, go to the hospital dermatology department for diagnosis and treatment. If necessary, go to the dermatology department of the hospital for consultation and treatment.