Bitter taste in mouth and black poop?

See your doctor. Black, tarry stools (poop) can mean blood in the stool. These symptoms together could mean a bleeding ulcer. See your doctor right away!
Reflux / bleeding. Reflux can lead to a bitter taste in the mouth. If reflux is severe it can lead to an inflamed esophagus. Also ulcers can form in the intestines from acid which can bleed. Blood from the stomach usually turns black when it passes in stool. Get this checked to be sure.