I have had diarrhea for several weeks now accompanied by loss of weight and loss of appetite. Soon after this began I had a colonoscopy and upper endoscopy. Both were completely clear. I have also had a stool study to detect blood. This was also negative. I am a 76 year old woman. The diarrhea is usually black and looks like coffee grounds; sometimes it is brown and is the same consistency as coffee grounds. It is very watery and is often explosive.