The more common clinical causes of thumb darkening include the following two: First, inadvertent contusion of the patient’s thumb may cause superficial soft tissue capillary rupture, and will be accompanied by local blood stasis formation, bruising in the process of absorption will lead to the color of the thumb darkening. Secondly, arterial occlusion of the upper extremity will block the blood flow back to the patient, thus causing local location ischemia and hypoxia, and the local skin temperature drops, making the thumb darker in color. If the color of the thumb is darker, you should go to a regular hospital for an examination. If the thumb is darker due to contusion, the condition is mild and can be relieved with proper rest. If it is due to arterial occlusion of the upper extremity, it is more dangerous and must be treated in time, otherwise it may cause serious harm to the body.