Some patients with constipation may have their stool accumulate in the residual tract for a long time, which may lead to darkening of the color. If the occult blood test is negative, there is no need to worry, as the symptoms can be improved by improving the diet and bowel movement, eating more vegetables and fruits, drinking more water and having regular bowel movements. If the occult blood test is positive, it means that the black color is caused by bleeding, mostly from the upper gastrointestinal tract, and the patient needs to undergo gastroscopy to clarify the cause of bleeding.