What Causes Black Coloured Bowel Movement?

Three months ago, I changed my diet dramatically....Smoothies for breakfast, very light lunch of salad and fruit and mostly a soup for dinner or grilled chicken on rare occassions...have never eaten so lightly before in my life. Been pretty religious about it and have dropped about 20 lbs. I ve always been regular with my BM s....2 weeks ago, I ate some foods that did not like each other....at all. Since then, I ve had difficulty forming normal stools...having mostly loose and running movements. What has me most concerned, is that the bowl contents are black in color. Again, no firm stool...more like a gravy but black in color. Been taking over the counter stuff Pepto Bismol and Imodium, but it hasn t prevented the off and on diarrhea that I ve been experiencing. My instinct tells me I need to get with a gastroenterologist pretty immediately. Your input please?