Thanks for posting your query.
Dark stools could be due to following reasons.
1. Bleeding in the upper gastrointestinal tract. This means it could be in the esophagus
, stomach or the proximal part of the intestines. The causes could be due to an ulcer, erosions or in the worst case a malignancy in the upper gastro intestinal tract.
2. Another common reason for passing dark stools would be the intake of iron supplements
3. Inclusion of certain foods in the diet like greens, blueberries etc. can also cause dark stools.
It is essential to rule out pathological cause like bleeding before attributing the dark stools to medicines or diet.
Certain facts in his medical history would point towards bleeding:
(a) Pain in the abdomen.
(b) Previous history of acid peptic disease
(c) Loss of appetite
(d) Loss of weight
(e) Recent intake of non steroidal anti inflammatory drugs like brufen
It is important to get your father seen by a gastroenterologist. He will first do a stool examination for blood. He may also suggest upper GI endoscopy
and a colonoscopy
Hope I've answered your query.