Hello,
Stomach ulcer can cause black/ dark stools but not only. Another possibilities includes:
diverticulitis , hemorrhoids , colon cancer and anal fissure.
Stomach ulcer can be caused by
Helicobacter pylori (bacteria) so there is a need to do tests used to determine whether H. pylori infection is present. These tests are: breath test, blood test or stool test but the most important is endoscopy where doctor may take tissue samples.
If bacteria is present doctor will prescribe Your mother 2 antibiotics like
Clarithromycin, Amoxicillin, Tetracycline or
Metronidazole and proton pump inhibitor. Also H2- blockers may help relieves gastritis pain.
Your mother should avoid coffee which stimulates gastric acid secretion, alcohol and irritating foods. Smoking increases the risk of peptic ulcers. Try to avoid spices. Your mother should eat frequently small quantity of meal.
Hope I have answered your question. If you have any further questions I will be happy to help.