Hi Thanks for posting your query on HCM. I appreciate your concern for the black watery stools. Incomplete data-age, gender, any medication intake. do you have vomiting of blood, stomach upset, do you smoke? The most common cause of black colored stool is bleeding mainly in the food pipe and stomach. Other causes include oral iron supplements, smoking and eating foods that are black or dark purple like grape juice, dark chocolate cookies and black licorice. Black stools due to food ingestion are not a cause for concern. However the other causes some of them may be serious like bleeding should be ruled out by relevant examination like stool analysis, etc. You are therefore advised to visit your gp for further evaluation and treatment. Hope this answers your query.