What Causes Black Stool?

Question: I am an 83 yr old female and in generally good health, some arthritis in my hands and knee. I take 3 medications, Benicar for high blood pressure 40/25, noritryptllene and klonopin to relieve nerve pain in my face, around left eye and down to check and lip. I had a moped accident about 20 years ago and have been on this protocol ever since after visiting about 6 different neurologists looking for a solution. The pain started after surgery to correct a tripod fracture, one surgery to insert a metal plate and another 9 months later to remove it because of extreme pain. I was told by neurologists that a nerve had been injured and there was little chance of the problem going away. That's the history. Here is the problem. Recently I developed dark stools, almost black looking to greenish. I had a colonoscopy which showed I have severe divertoculosis which does not bother me. Doctor advised diet changes. I have had blood tests which do not disclose any problems. I am waiting for the results of the latest blood test. My doctor did a test for blood, no blood in stool. I do take a lot of vitamins and minerals not under the advice of a doctor. Recently I decided to go cold XXXXXXX and eliminate all supplements. For a day or 2 I had normal stools brownish. I decided to slowly introduce supplements again, taking B12, Vitamin D3, and Liquid Omega 3. The stools are dark again. My blood tests always show inflamation readings as high as 40. I am beginning to attribute this to arthritis and internal inflamation. Naturally, I worry about some new problem developing, although my GP seems to think, at this point that nothing is wrong. He suggested the colonoscopy, which I did and stress test which was ok, and test for valve issues and aorta problem. My upper aorta is 4.0 in size vs. 3.8. 2 years ago I was diagnosed with 4.4. I have previously been diagnosed with floppy valves, and had arythmnias for many years. Don't suffer that any,more. I know there is a lot here, but I am very concerned about the abnormal bowel color. I have 5 grown children and11 grandchildren and do get stressed very easily. I just retired this year. My oldest daughter has anagressive breast cancer and has just finished 4 chem treatments of A and is now receiving the 2nd treatment of Pax?? . My husband who is 88 tells me to relax I worry too much........ I know this is a long story. My main concern is the stool problem which is several months old. XXXXXXX B,