Hello XXXXXXXXX,
Black stool can be indicative of blood high in the intestinal tract. I would suggest checking the dog's gums and making sure that they are nice and pink. If they are very pale or white, then you will need to get him into a vet tonight. If this were the case he would probably also be lethargic or weak.
For treating diarrhea at home with dogs, the most common recommendation is to fast the dog for 12 - 24 hours ( he may be doing this himself). Then feed a bland and bulky diet of boiled chicken and rice (2/3 rice) for several days. Then take another couple of days to slowly transition back to normal diet. I have found immodium to be most helpful for diarrhea in my dogs. The normal dosage is the normal 2mg tablet for a 50 lb dog. I would not exceed the 4mg recommende for people. Another dose can be given in 4 - 6 hours if the dogs is still having diarrhea. It is usually effective in one or two doses. Here is a link to more information.
http://www.peteducation.com/article.cfm?c=26+1303&aid=1432
If the dog is acting lethargic or painful, doesn't respond to OTC medications, continues to have diarrhea, is still refusing to eat after the fasting period or develops new symptoms I would take the dog into the veterinarian. Having a stool sample checked for intestinal parasites would be an excellent idea if this continues. They may also want to do some x-rays to look for possible blockages.
If you need more information just reply to your question.
Donna P.
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Veterinary Technician
Veterinary Technician with 15 years experience. Boston Terrier Breeder