hello thanks for posting on HCM, Helicobacter pylori infection initially causes gastritis. which is mild in nature initially. in mild gastritis you will get burning type upper abdominal pain which is aggravated or relieved by food, nausea and feeling of stomach fullness, belching\ burping, and sometimes even malaise. however in order to have blackish stool there should be gastric ulcers which is not in early period. coated tounge is not an usual symptom of gastritis instated it may reflect some vitamin B deficiency. however H. Pylori infection causes recurrent gastritis with gastric ulceration with time and finally gastric cancer. if you feel mid abdominal pain , blood stool and indigestion it may be inflammatory bowel disease and not purely gastritis. coated tounge is also a feature of it. Its better if you get checked yourself. thank you.