If you have black feces, let it sit in the toilet, then move it around. If the water turns dark red, it's blood. Greenish black is from iron or can be from food sources such as green/blue dye (Fruit Roll-ups make kids poop dark green). Dark black poop from blood is from bleeding high up in the body. Fresh red blood in poop is from closer to the anus, like hemorrhoids or fissures.
If it happens again, save a sample for analysis just to be safe. And your next physical should include a fecal blood check.
If it happens again, save a sample for analysis just to be safe. And your next physical should include a fecal blood check.