First, patients with hyperlipidemia will also have black blood because of the increase of blood viscosity. Second, it is common in chronic hypoxic diseases, such as chronic obstructive pulmonary disease and congenital cyanotic heart disease. Due to long-term tissue hypoxia, the arterial blood content in the blood is low, and blood may turn black. Third, it is a disease, that is, nitrite poisoning, which is also called enterogenous cyanosis in clinic. Because nitrite oxidizes ferrous iron in hemoglobin into trivalent iron, methemoglobinemia will occur and blood will turn black.