What causes a perianal cyst to turn black?

If perianal cysts appear black, it may indicate a problem in the digestive system, possibly due to gastrointestinal bleeding. Bleeding from the upper digestive tract often results in darker, black-colored blood that mixes with stool. This is commonly seen in conditions such as esophageal variceal bleeding. The characteristic of such hemorrhage is that red blood cells are broken down by digestive fluids—such as gastric and duodenal juices—and decomposed into non-blood organic substances. These substances then mix with digested food and enter the small intestine and colon for further digestion and absorption.