What causes black blood in stools?

Ulcerative colitis

Ulcerative colitis is a chronic non-specific intestinal inflammatory disease, due to long-term stimulation of inflammation leading to colonic mucosa congestion, edema, or even damage, bleeding, resulting in the above symptoms.

3, intestinal tumor

if the occurrence of intestinal tumors, tumor growth, reproduction process may destroy blood vessels, resulting in bleeding, and then cause the above symptoms.