Black and purple capillaries on the top of the feet may be caused by lower extremity venous thrombosis, trauma, varicose veins and other factors. 1. Lower extremity venous thrombosis: when venous thrombosis occurs in the lower extremity, it will lead to poor blood flow in the dorsal vein of the foot, and the oxygen content in the vein will be reduced, which will lead to the symptom of black capillaries. 2. Trauma: when the foot is hit, squeezed and other traumas, it may lead to capillary rupture and bleeding, blood stagnation, resulting in the symptom of blackening of capillaries. 3. Varicose veins: excessive pressure on the lower limbs, long-term standing and other factors can lead to varicose veins. When varicose veins appear on the back of the feet of the patients, it will lead to blood circulation obstacles and the appearance of black and purple capillaries. It is recommended to go to the hospital in time to find out the cause of the disease and have relevant treatment in time.