When the arm turns blue after the blood is drawn, it is mainly because the bleeding area is not pressed tightly after the blood is drawn, resulting in local bleeding and the blood is absorbed in the subcutaneous tissue, forming bruised patches. Therefore, after the blood is drawn, when the needle is pulled out, the bleeding area should be pressed tightly, either with a cotton swab or a cotton ball, and the pressure needs to be more than one minute, not rubbing back and forth, not moving back and forth, because it may cause the pressure to be unconsolidated, resulting in local bleeding and bruising. If there is local bleeding and a lump, you should re-press and elevate the arm if necessary, which will effectively stop the bleeding. If bleeding should be applied with ice as cold as possible to reduce the leakage of surrounding tissue fluid. If a local bruised patch forms after two days, you can use a hot towel to apply a hot compress or take a hot bath to accelerate the blood circulation around the area so that the bruise dissipates more quickly.