https://www.vetinfo.com/causes-dog-t...oloration.html Black Tongue
One of the few breeds of dogs that does not have a pink tongue is the Chow Chow. Chow Chows have black tongues. If you have a dog breed that should have a pink tongue, take him to a vet if it is black. "Black tongue" occurs due to ulceration or inflammation in the mouth along with blood-stained drooling saliva. The dog's breath also turns foul. Black tongue can also be due to a deficiency of niacin. This tongue discoloration illness can become fatal if not treated in time.