Friend, there are certain breeds that will have a black tongue or spots that are black on their tongue and this is completely normal (see list below).
Now, if the tongue was not or never had a black spot this darkening could be what is called "hyperpigmentation." This usually occurs secondary to an inflammatory process as a result of infections, allergies, etc. What happens is that inflammatory cells (macrophages) release melanin during the inflammatory response, the melanin causes the pigment to turn black. The black color will likely not go away, it is not harmful just a consequence of inflammation. Finally, we can sometimes see hyperpigmentation with certain types of tumors like Melanoma.
In this case, I would recommend having family veterinarian examine his tongue to rule in a normal pigment vs a pathological condition as described above.
Please follow the link below so you can see breeds of dogs with a black tongue or spots that are black.
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