There are two reasons why blood is found to be dark when it is drawn: First, when blood is drawn, venous blood is usually drawn, and the oxygen content in venous blood is relatively low, the concentration of oxyhemoglobin is low, and the blood itself is dark black. If the bleeding is drawn in the morning on an empty stomach, due to the lack of water during the night, the blood concentration will also appear dark in color, so when the blood is drawn, the blood color is dark, generally speaking, is a normal physiological phenomenon. If there are no obvious symptoms such as dizziness, dizziness, tinnitus, etc., there is generally no need for special treatment. Second, if the blood color is particularly black when the blood is drawn, it is likely to be methemoglobinemia caused by nitrite poisoning, this situation requires timely treatment, generally after vitamin C and methylene blue treatment, the blood color black will be corrected. At the same time, you can also test blood lipids, blood sugar and other indicators to see if there is an increase in blood viscosity.