It is possible for your motions to become darkened if you are taking certain medicines - iron tablets can have this effect. However, as suggested above I would advise you to seek medical advice. If the cause is due to internal intestinal bleeding then the medical name for this effect is melena and you may wish to google this word in order to find out more for your medical consultation.
regards
Morecambe
Yeah, black, tarry stool means you have blood in your stool, and it could be from anywhere. You need to see a doctor immediately.