Hi there,
The major risk factors for colon cancer are genetic factors (if it runs in your family) and old age so you seem too young to experience colon cancer if there is no family history of it.
That saying black stools are an indication of bleeding in your stomach or bowels which is a worrying sign but in someone as young as you, it is more likely to be a bleeding ulcer or inflammatory bowel disease. Inflammatory bowel diseases are ulcerative colitis and Crohn disease which are associated with inflammation and swelling of the lining of the bowels and are associated with black, tarry stools with mucus. See a doctor who will probably take a blood test to check for anaemia and arrange a colonoscopy.
Best wishes.
Have you been to a doctor for this? Black stools can be indication of GI bleeding and I really think you need to get to a doctor.