What kind of sickness causes my diarrhea to be black?

GI Bleed or drugs. If u r bleeding in your stomach this could cause ur stool to be black (melena). It also smells very bad. Peptobismol can also make your stools black. If u r not taking any drugs and having stomach pain with the black stools u should see a doctor.
Blood, pepto bismol (bismuth subsalicylate) Truly black diarrhea is concerning for digested blood and evaluation a doctor is recommended. Dark green diarrhea is more common and related to bile in the stool. Otherwise the color of stool is related to the colon of whatever is consumed. Also, bismuth containing anti-diarrheals (pepto bismol, kaopectate) make stool black.