I agree with MoRita, black tarry stools are almost never a good sign..from pediatrics, to adults, to dogs! Watch this dog closely...this can mean a bowel obstruction, or an ulcer, or even worse a form of cancer. Pure bred, small dogs can be finicky with skin disorders as well as other diseases, and should be cared for closely, and collaborating with your vet to ensure it's health. I would take your dog in tomorrow just to be sure, because like MoRita said, it can get ugly fast. Treat it like a baby whose body systems can shut down fast if they don't have adequate nutrition as well as elimination. Hope this helps, and good luck!