Colon cancer, also known as colorectal cancer, is a cancer that occurs in the large intestines. According to the Mayo Clinic, colon cancer often starts out with benign cancer cells that turn malignant over time.
One of the symptoms of colon cancer is blood in the stool. The blood can either be bright or dark red. When it is dark red, it turns the stool a black tar-like color.
Medicinenet.com says colon cancer can be developing for years before it is detected. The right side of the large intestine has a spacious storage area that can allow a tumor to grow to the size of a baseball before it is detected.
Black or bloody stool is often an early symptom of colon cancer, and the North Shore Medical Center encourages people who experience blood in the stool to get to a doctor immediately.
Bright red blood in the stool can be a sign of tumors in the left side of the large intestine, and black stool normally indicates a tumor growth in the right side of the colon.
According to the North Shore Medical Center, if colon cancer is caught early enough, successful treatment is possible.