The blackening of the joints of the hands may be due to trauma, chronic dermatitis, diabetes, osteoarthritis and so on. 1. Trauma: the blackening of the hand joints may be due to trauma, such as after the hand joints are pinched or smashed, there is subcutaneous bruising, ischemia, or tissue degeneration and necrosis in the injured area, resulting in blackening. 2. Chronic dermatitis: if the patient suffers from chronic dermatitis, due to the local inflammatory reaction, resulting in an increase in melanin on the surface of the skin of the finger joints, resulting in the phenomenon of blackening. 3. Diabetes: Diabetes is a chronic disease that may lead to blood supply disorders in the joints of the hands, resulting in localized bruising or ischemia, and blackening of the joints of the hands. 4. Osteoarthritis: due to excessive strain, age factors, etc., resulting in degenerative diseases, such as degeneration of articular cartilage, bone spur formation, etc., caused by osteoarthritis, there will be joint pain, joint congestion or swelling, and there may be blackened joints and other phenomena. When the joints of the hands are blackened, you should consult a doctor in time to clarify the cause of the disease and then treat the cause of the disease.