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I am sorry to hear you are worried about Otis. I am a veterinarian and will do my best to help.
The black diarrhea is very concerning. When the stool is black, it may indicate internal bleeding. If you look at his gums, are they bright pink in color, or do his gums look pale? If his gums look pale pink or white to you, internal bleeding is even more likely.
He may be weak, and that is why he is lethargic and stumbling.
I am very worried about him. If he were mine, I would be looking at the diarrhea under a microscope to see if there are blood or parasites in it. I would do a test of his red blood cell count to see if he is anemic. I would consider testing for parvo virus as well, a viral disease of puppies. He can still get parvo after a vaccination; a dog is not fully protected until they have the full series of vaccinations.
Let me know what other questions you have. I think he should see a veterinarian, as soon as possible, especially if his gums look pale.
You are welcome. Let me know how he is doing.
Kindly consider rating my answer (always appreciated); you can still get back to me with updates and questions after rating. I hope he is doing better soon.
How is Otis doing?