Darkening of the blood means that the blood appears dark red. There are various reasons for dark blood, the most common ones are: i. When blood is drawn in the morning on an empty stomach, the blood may appear dark because it has not eaten or watered during the night and the blood is concentrated. Second, patients with hyperlipidemia, due to the increase in blood viscosity, may also have black blood. Third, common in chronic hypoxic diseases, such as chronic obstructive pulmonary disease, congenital cyanotic heart disease, etc., due to long-term tissue hypoxia, the blood among the arterial blood content is low, will also appear the blood black. Fourth, is a disease, namely nitrite poisoning, also clinically known as enterogenic cyanosis, due to nitrite oxidation of divalent iron in hemoglobin to trivalent iron, appearing methemoglobinemia, will also appear black blood.