Yes the black colored stool may indicate that he has upper intestinal tract bleeding going on - unless of course you have been feeding charcoal dog biscuits.
With this age dog I'd suspect possible swallowing of something sharp or made of cloth etc that has irritated and perhaps damaged his intestine somewhere. If you are missing any underwear, toys, string, bones, sticks etc that might be the problem.
Another possibility is a bleeding ulcer from a helicobacter infection.
I would bring a stool sample with you if you can and see a vet if at all possible today.
If you cannot do that you can see if Tagamet or Pepcid will help. You can read about using these in dogs here including doses and cautions
www.petplace.com/drug-library/cimetidine-hcl-tagamet/page1.aspx
www.petplace.com/drug-library/famotidine-pepcid/page1.aspx
Inform the vet if you use one of these when your dog is seen.
Hope this helps you!
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