Hello!
Thank you for the query.
Tar like stool usually indicates bleeding from upper GI tract including peptic ulcer,
stomach cancer or
small intestine malformation. Also iron intake can cause black stool. However if you do not take any, bleeding is the most possible reason.
If your pain is in the upper middle abdomen, nausea appears, peptic ulcer seems most probable.
Constipation can be caused by bad diet (low fiber intake),
hypothyroidism, medicines (ca-blockers, antidepressants), bacterial infection or even bowels obstruction due to a cancer or inflammatory bowels disease.
Both cancer and inflammatory bowels disease can also be a source of blood in the stool.
I suggest you to do not ignore your symptoms. Please visit your doctor and have blood work (low hemoglobin will indicate bleeding),
abdominal X-ray (will rule out obstruction) and abdominal ultrasound. In case of bleeding, gastrosopy should be done.
Hope this will help.
Regards.