While certain medications can lead to changes in the color of poop, antibiotics specifically do not tend to cause black discoloration of poop, although it can lead to gastrointestinal upset. If the diarrhea and inflammation in the gut is severe, it could lead to blood in the stool. And again, depending on where the blood comes from, may make the poop look red or black. Other drugs can directly affect the color of poop. While not used commonly, things like Pepto-Bismol can cause darker or even black colored poop. (Warning: Never give your dog medications meant for humans unless directed by your veterinarian.)