Because it's the natural color of his tongue
There are a few dog breeds that have naturally blue tongues. Sometimes they are even quite dark and may even appear black.
Chow-Chows and Shar-Pei are the two dog breeds best known for having naturally blue tongues.
As with all physical characteristics, the reasons for this particular color are genetic and are linked to a mutation transmitted to offspring. Some dogs crossed with dogs of these breeds may also show blue to black spots on a pink tongue.
Because it lacks oxygen
The color of a dog's tongue can tell a lot about their general he alth and the state of their blood circulation.
A bluish coloring of his tongue and mucous membranes, when it is not natural in your dog, must therefore make you think of a lack of oxygen in his tissues, which is also called hypoxia in medical jargon.
A heart or respiratory disease or disorder is usually the cause of hypoxia in dogs. Cyanosis (bluish discoloration of the mucous membranes) can thus typically occur following heat stroke, drowning, hypothermia (significant drop in body temperature), pneumothorax following trauma, poisoning, smoke inhalation, choking by a foreign body etc.
All of these situations are absolute veterinary emergencies and require immediate veterinary care.
Because he ate blueberries
Your dog can also have a blue tongue simply because he ate a food that colored his tongue blue or dark purple. All foods rich in anthocyanins, naturally blue-violet pigments, can stain the tongue very strongly. This is particularly the case with berries such as blackberries or blueberries which are safe for your pet!
If it's a non-toxic dog food, there's no need to worry. Your dog should have a normal colored tongue within hours.
On the other hand, if you think your dog has eaten a toxic food, contact your veterinarian without delay.