Thank you for the query.
Black stools (tarry like) and abdominal pain
is most likely caused by bleeding from peptic ulcer
. I do not know if you have been tested for Heliobacter Pylori, but if not, you should so, and antibiotics should be ordered in case of positive result. It seems like Omeprasole does not work for you. It happens for some people and in such case, other PPI medicine should be prescribed as it may have better effect.
But now you should definitely visit your doctor as soon as possible and have upper GI endoscopy
performed to rule out any ulcer bleeding. Blood work should be also done.
Hope this will help.