hello there. there is normally a certain amount of acid present in the stomach and that is HCl. the acid protects from certain bacteria or other things which may be ingested through food. also, there is something called a lower oesophageal sphincter. which acts as a barrier between stomach and oesophagus. when you have lots of acid production in your stomach, or there is a problem with the lower oesophageal sphincter, you may have acid reflux. which will present as heartburn, chest pain etc. the causes of increased acid secretion are- oily and spicy foods, stress, irregular food habits, certain medicines etc. you need to get a endoscopy done for your acid reflux. colonoscopy would be for your blackened stools. you need to avoid oily, spicy foods, drink lots of water, have proper food timings, instead of having 3 large meals, have 5 smaller meals, don't sleep immediately after meals, practice yoga/meditation. this advise should help you keeping the acid levels normal. if your problem still persists, consult a gastroenterologist and get a endoscopy done and then take it from there. regards