by 
The Curious Bag
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If your dog's tongue develops a coating, spots or other discoloration, schedule a trip to the vet immediately. These changes can be symptoms of serious, even life-threatening conditions.
Dye Transfer
A dog's tongue may pick up a coating of dye from chew toys. Dyes may be poisonous; check with your vet.
Blue Tongue and Gums
Cyanosis, or blue mouth discoloration, indicates low oxygen levels in the blood due to respiratory disease, heart disease or toxin exposure.
Spots
The appearance of a dark spot, especially with an irregular border, may indicate a melanoma. Pale spots may signify that your dog has been licking objects with irritating chemicals on them. They may also indicate an immune system problem.
Black Tongue
A black tongue accompanied by bloody drool and unusually bad breath may indicate ulcers or inflammation (stomatitis) in the mouth. These may, in turn, be symptomatic of a variety of potentially serious conditions such as thrush, trench mouth, uremia or distemper. Black tongue can also be a symptom of a niacin deficiency.
When a Black Tongue is Normal
The tongues of Chows, Shar-Peis and other dog breeds can be black or black-spotted naturally. See "The Black Tongue Debate" for details and pictures.
Source:
VetInfo4Dogs.com: Possible Causes of Dog Tongue Discoloration
MedicineNet.com: Sore Mouth (Stomatitis) in Dogs
iVillage.com Symptom Solver: Tongue Spots
More Information:
Chow Club Welfare Committee: The Truth About Those Black Tongues
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