There are various causes for black tarry stool (presence of the digested blood) including bleeding oesophagus (or anywhere else in the gastrointestinal tract), infections, parasites, poisoning, medicine side effects, tumours, foreign bodies (and other obstructions) or other conditions. (wagwalking.com)
Black tarry stool shows bleeding in the gastrointestinal tract. (dog-health-guide.org)
Black tarry stools mean that she is bleeding somewhere in her upper GI tract. (wagwalking.com)