hi there.. thanks for your query... well actaually the black particles in stool is due to dried blood that is due to bleeding in upper gastrointestinal tract(stomach , oesophagus, or duodenum
).. its called malaena.. cause may be bleeding peptic ulcers, gastritis
, esophageal varices(caused by problem in liver
). it may also be due to medications like aspirin
which is commonly prescribed in cad patients.. you must be examined and investigated soon to rule out any hidden potential disease.. better consult a doctor regarding this matter soon.. hope that wil help you..