Hello, my name is***** and I am happy to help you and MacGregor. I have a few questions to better assess the situation.
1. Any vomiting?
2. Are we eating ok?
3. When you lift his lip, do his gums look pink or pale?
4. Any recent senior lab work from your vet?
Dark, tarry diarrhea occurs when a pet has mild bleeding occur in the small intestine. The blood gets digested and passes in the stool. There are a multitude of causes (intestinal parasites, stomach ulcer, irritation to the intestines from eating something they shouldn't, all the way to organ issues).
I typically start with a poop sample to rule out parasites. In a 15 year old patient, I'd discuss doing some basic senior blood work to make sure the liver, kidneys and pancreas are functioning ok. From there, I would prescribe some medication to calm down the intestines and have you feed a bland diet (boiled chicken and rice for example). You and your vet may not figure it out on the first go round, but they should lay out a systematic approach if the dark diarrhea does not respond to initial treatment.
If I have answered your questions, please kindly rate me (this is how I am compensated for my time). If not, please reply back and I am happy to answer additional questions. Thank you.