If you have blood in your stool it will usually be very black or you could actually pass red blood...If you are having any stomach pain that you cant explain then you should probably let your dr. check you. One thing to remember if you have been taking pepto bismol or any antacid then it usually will turn your stools black....Something in the medicine causes it to turn colors...If that is the case then stop taking them for a week and see if your stools clear up....But i would still call the DR..best wishes..:)
First of all determine if it could be something you are eating that your body isn't completely digesting. Blood from high up will turn your stool black. Any change in bowel habits should be evaluated, but look at your diet first.