Black colour in the stool might be due to the presence of blood and is called melena. During this condition, blood that originates from the upper parts of the digestive system such as the mouth, throat, oesophagus, stomach and duodenum can turns black when the blood comes into contact with stomach acid. Bleeding from some of the upper parts of the respiratory system such as the nose can also cause black stools if the blood is swallowed. Conditions like peptic ulcers, stomach cancer, nose bleeding, oesophageal haemorrhoids can cause melena. To diagnose the condition, you can request laboratory tests to confirm the presence of blood in the stools. This is the frist step ... and you can consult and internal medicine specialist for advice and consultation.