 [ Symptoms ] diarrhea for 4 wks watery and then black stools now a redish color but not like blood and mucus is this serious saw dr waiting for stool culture had stomach xrays a lot of gas waiting for ins to get ct scan [ Disease ] black stools could indicate blood but it can also mean that you have been taking pepto bismol the bismuth in this medication causes that stools to be blackish red like stools do not necessarily mean blood but it could be there are special stool tests that will test for the presence of occult hidden blood that should be done four weeks of diarrhea does require a thorough work up so i am glad that this is being done