Colorectal cancer
Colorectal cancer causes blood to accompany your stool. Depending on how much blood there is, it may only turn your stool black. The Centers for Disease Control and Prevention [7] (CDC) says that colorectal cancer, the word for cancers of the colon and rectum, is the third most common type of cancer.
Signs that you may have this cancer beyond having black or bloody stool include changes in bowel movement, loss of appetite, weight loss, and fatigue. Like most cancers, the symptoms may not appear in the early stages of the disease. This is why regular checkups are important.