Hi and thanks for the query, It is prudent for sure to think that the dark stool was blood. Especially if the stool color was different before the incident. A history of peptic ulcer disease is important to be noted and also any notion of gastroesophagal reflux disease. A keen evaluation by a gstroenterologist is the ideal thing to do. While waiting, taking antacids like Alluminium Hydroxide and Proton pump inhibitors like Omeprazole could be helpful as you await to see your doctor. Kind regards. But its urgent to get this consultation done.