The causes of the dark color of blood drawn from children include the following: First, normal physiological phenomena. Children’s blood is collected from a vein, which contains a lower concentration of oxygen and will have a dark color. Secondly, caused by children suffering from hypoxic diseases. If a child has a serious lack of oxygen, such as an acute attack of asthma or severe pneumonia, the oxygen concentration in the child’s body is low, which can also cause a dark color. Oxygen therapy can be given to the child in time to improve the oxygen status, otherwise the low oxygen level in the child may cause tissue damage.