Black colored stool may be due to:

FOOD

  • Beets
  • Blueberries or blackberries in large amount
  • Licorice
  • Raw or undercooked red meat, or other foods with substantial amount of animal blood, like Black pudding

BLOOD

Any blood passing through the stomach will be colored black, due to oxidation of hemoglobin iron by stomach acid. Some stomach acid may appear in duodenum, so any bleeding arising from duodenum, or above, will cause black, tarry, foul smelling stool that sticks to the toilet. Possible causes of black stool from bleeding include:

  • Stomach or duodenal inflammation (gastritis), erosion, or ulcer due to alcohol, smoking, strong spices, or infection with H. pylori bacteria
  • Stomach or duodenal cancer
  • Esophageal varices due to liver cirrhosis (often in alcoholics), or portal hypertension
  • Mallory-Weiss tear (a tear in the esophagus from violent vomiting)
  • Duodenal ulcer, or duodenal inflammation (severe celiac disease) (Some stomach acid may appear in duodenum
  • Small intestine: Meckel’s diverticulum (an inborn defect, where a part of stomach mucosa appears in the small intestine pouch, secretes acid and often bleeds)
  • Nose or para-nasal sinuses (high blood pressure, tumor, injury)
  • Mouth (tooth extraction, injury)
  • Throat (severe inflammation, cancer)
  • Lung, bronchi, or trachea (cancer, tuberculosis, rarely heavy pneumonia)
  • Spontaneous bleeding into the gut due to thrombocytopenia, hemophilia, metastases, or sepsis

MEDICATIONS

The following medications, taken by mouth, may color the stool black:

  • Charcoal
  • Iron supplements color stool black or green
  • Pepto-Bismol (bismuth subsalicylate, an anti-diarrheal drug)
  • Any medication that can cause stomach bleeding: aspirin, ibuprofen, naproxen, and other non-steroid antirheumatic drugs (NSAIDs)
  • Vanadium products, often used by bodybuilders

POISONS

  • Lead

OTHER

Black pepper-sized specks in stool may be due to clumps of bacteria, often in small intestinal bacterial overgrowth (SIBO). 

Black Diarrhea

Black diarrhea may be due to:

  • Medications/supplements: anti-diarrheal drug Pepto-Bismol, iron pills
  • Bleeding from the stomach or upper small intestine during diarrhea

Further Reading:

Normal & Loose Bowel Movements
Find a Causes of Loose Bowel Movements From Medical Symptoms
Bowel Anatomy, Motility, Digestion & Gut Flora
Fructose Malabsorption
Yellow Bowel Movement
Green Bowel Movement
Mucus In Bowel Movement
Foods to Avoid in IBS


Further Reading :
  • Is Diarrhea Contagious? How Can Be Diarrhea Prevented?
  • Bowel Bleeding ~ Medical Terminology
  • Melena ~ Medical Terminology
  • Blood in Your Stool & Fecal Occult Blood Test (FOBT)