Darker stool might be caused by presence of disintegrated blood in the stool. This blood usually origins from the upper gastrointestinal tract. The blood from the lower gastrointestinal tract is usually not completely digested and is visible as reddish dots on the stool. Having in mind that you already have a history of gastric problems, you might be experiencing stomach or duodenal ulcer that started bleeding a little. Simple laboratory tests for blood in the stool might confirm or rule out this hypothesis. If the results are positive, your doctor might refer you to a gastroenterologist for further tests in order to have a complete picture of the condition in your stomach and the recommended treatment afterwards.