The common reasons for dark blood are: First, the blood is usually drawn from a vein, and the oxygen content in the blood is low, so it is easy to appear dark red, while if the blood is drawn in the morning, due to fasting at night, the blood is concentrated and the blood flow is slow, it may aggravate the lack of oxygen in the blood, so the color of the blood will further darken. Second, if it is some long-term chronic hypoxic diseases, such as residents of highland areas, chronic obstructive pulmonary disease, and congenital cyanotic heart disease, etc., due to long-term hypoxia, the blood will also appear black. Third, nitrite poisoning, also known clinically as enterogenic cyanosis, will be after nitrate poisoning, the oxidation of divalent iron in hemoglobin to trivalent iron, and the combination capacity with oxygen is reduced, which will cause the symptoms of severe tissue hypoxia. The blood will appear distinctly dark black, the color of ink, clinically called enterogenic cyanosis, and requires vitamin C and methylene blue treatment.