The most likely cause of black stool if you are not feeding charcoal biscuits is intestinal bleeding in the upper gastro intestinal tract.
This could be from infection, swallowed non food item, any medications he is on, or a bleeding issue brought on from ingestion of rodent bait or even a hereditary bleeding issue.
It might also be from pancreatic inflammation which you can read about here
Or a condition called Canine Hemorrhagic Gastroenteritis (HGE) which you can read about here
So seeing the vet to find out what is going on should be a priority.
An over the counter med you might try would be Pepcid or Tagamet or Zantac.
Be sure to tell the vet when you see them if you use any of these.
You can read about dose amounts (and cautions) scrolling down on the pages here and just close any pop up windows to see all the info
These work better than pepto and are formulated for controling gastric bleeding such as ulcers.
If that helps then you can see if some bland food and plain pedialyte added to the water bowl will help.
I would suggest trying boiled chicken meat or hamburger with the fat drained off, mixed with really well cooked rice in a ratio of 1/3 meat to 2/3 rice to see if that works. It's not a balanced diet but OK to use for a few meals while working on controlling digestive upset.
If your dog might have gotten into anything toxic, appears dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has a rectal temp over 103.5F (normal is 100.5-102.5 Fahrenheit or 38 - 39.2 Celsius ), has gums or tongue that look white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, or is becoming lethargic and non responsive to you then you may want to get the dog in to see an emergency vet if one is available in your area.
This site may help you locate an emergency vet if you need one
Hope this helps you!