I'm not a doctor, but my gastrointerologist told me that dark stools can be an indicator of dried blood in the stool. I suggest you see a GI specialist ASAP. He or she may recommend a fecal ocolt blood test, a sigmoidoscopy or a colonoscopy.
Hi,
The character of the stool is generally related to what you eat and whatever medications or supplements you are taking. Your description is not really specific for anything, and as you are writing on a cancer forum, it doesn't raise any alarm for cancer. The presence of blood is one of the basis of screening tests for colon cancer, and based on your description of it now being black and tarry, then there probably isn't any blood.
Iron supplements tend to make the stool darker. You could discuss the need for the supplements with your doctor and experiment by removing some of them to see if any of them are causing the change in the character of the stool. You could also make a diet diary to record what you are actually eating (it is hard to rely on simple recall alone) and show them to your doctor as well to identify if there is anything there that could be the potential cause.
There is a wide range of what is considered normal, so odds are what you are experiencing now is within the limits of normal still, so trying to change things may not really make any difference.