Without further information it is difficult for me to provide the best advice but I will certainly try.
Please reply when you can and we can continue the conversation.
Black stool can be a sign of bleeding high up in the gastrointestinal tract; in the stomach or small intestine.
This can occur due to e.g. an infection, ulcer, kidney disease, a blood clotting disorder or toxicity.
As he is also coughing, I'd want to rule out e.g. lungworm or another blood clotting issue.
If he has pale gums, a bloated tummy or is breathing fast, these would be red flags.
A vet visit is important so we can check him over and look into this further.
He may need some tests such as a blood test, stool analysis and chest x-ray in order for us to get to the bottom of this.
This window stays open so please feel free to keep me in the loop and to as any further questions you may have.
Wishing Basel all the best.