What Causes Black Stool After Having Iron Supplements?

I have been on iron supplements for two months. Stools were a dark almost black as to be expected but lately they are a normal dark brown. Is that a problem? I am a 65 year old female with a history of occasional anemia from a no obvious cause. I have been tested with colonoscopy, endoscopy and pill cam, all negative. The conclusion was a mild slow bleed from weakened walls most likely caused by years of NSAIDS before hip surgery. Does the brown stools mean the iron is working?