Black stool is usually a sign of internal bleeding but can indicate some other problems. It indicates older blood. Any history of renal or liver problems or is it possible your dog got into or was given something toxic in the last week or so (even OTC meds for us, that is a pretty common cause)? Your vet will be able to narrow down the possibility's after an exam and speaking to you. You should get your pup seen asap. I hope your baby is okay and feeling better soon.