The causes of blackened blood vessels mainly include chemotherapy, toxic stimulation, and vascular hypoxia. 1. Chemotherapy: when patients use chemotherapy drugs, if they are infused through the peripheral blood vessels, it will cause irritating damage to the walls of the blood vessels, resulting in stiffness and blackening of the walls. 2. Poisonous stimulation: For example, after being bitten by poisonous snakes, or when poisoned by drugs, the color of blood vessels and local skin will turn black due to the stimulation of toxins. 3. lack of oxygen in blood vessels: blood vessels turn black because the color of blood is too dark, the blood vessels may be blocked and unable to flow normally. Complete occlusion of proximal blood vessels can lead to ischemia and lack of oxygen in distal blood vessels, resulting in blackening. There are many other reasons for the blackening of blood vessels. If you feel uncomfortable, it is best to go to the vascular department of the hospital and other related departments to clarify the cause of the disease and treat it actively so as not to delay your condition.