A black bruise on the leg is a petechiae on the leg, which is a condition of bleeding under the skin on the leg. There are various reasons for the appearance of black bruises on the legs: First, in trauma, due to the external force acting on the capillaries, it will cause the rupture of the capillaries and the red blood cells will seep in from the vessels and leak outside the vessels, and the black bruises on the legs will appear. Second, can be seen in patients with thrombocytopenia, no matter which cause of thrombocytopenia, the human body bleeding symptoms will be obvious, capillary bleeding will occur, there will also be the symptoms of black and blue legs. Third, it can be seen in vitamin C deficiency, because when vitamin C deficiency, the density of human capillaries will decrease, permeability will increase, red blood cells may leak out of the blood vessels to the outside of the blood vessels, also appeared the black and blue legs. Fourth, seen in coagulation disorders, such as hemophilia patients, due to coagulation dysfunction, red blood cells from the blood vessels leaking out to the outside of the blood vessels, also appeared on the legs black and blue situation.