I recommend that you seek prompt medical assistance. There are changes in stool color due to eating certain foods, or bismuth medicines (Pepto-Bismol) or iron supplements. Green stool often occurs with severe diarrhea from a diet high in green leafy vegetables such as spinach. If you cannot attribute your stool color to these benign causes, you need to see a doctor
as soon as possible for a diagnosis. This is especially true if you've had other accompanied symptoms such as abdominal pain or bloating. A change in the appearance of your stools may signal an internal bleeding. Black stool
usually means that it is bleeding for some time from the upper part of the gastrointestinal (GI) tract, which is typically from the esophagus, stomach and upper small intestine. Blood will typically look like tar after it has been mixed with the body's digestive fluids as it passes through the intestines. The upper part of GI tract usually causes black and tarry stools due to peptic ulcer
disease, angiodysplasia (fragile, abnormal blood vessels), gastritis (inflammation of the stomach lining), or esophageal varices (abnormal, enlarged veins in the lower part of the esophagus), to name a few. Again, it is advisable to seek prompt medical assistance. If you have passed a lot of blood, you may also need emergency treatment. Good luck.