Whats wrong if my poop is black?

Depends. Dark stools may be caused by digested blood from a stomach bleed. A test for fecal occult blood should answer this. Some foods like blueberries or meds like pepto-bismol can also darken stools. See your doc and share any symptoms you have so you can get a good workup.
See md. Could be blood in the stools which could mean some serious illness. Also taking iron tabs can make stool black.
Danger. Black bm's should be considered digested blood until proven other wise. Sudden death from bleeding ulcers, slow death from cancer...Ect, get to your doc. They can diagnose this in a timely bases. There are benign causes, like excessive pepto-bismol intake.