A blue tongue is a typical trait of the Chow Chow breed. But the Chow isn´t the only breed: Shar Peis have blue tongues as well. And the FCI-standard of the breed Thai Ridgeback quotes “black marking in the pigmentation in the tongue is preferable”.
The underlying genetic variant hasn´t been identified so far, but it probably acts in an incomplete dominant mode of inheritance, because mixes of Chow Chow and other breeds have blue spotted tongues. More or less extended dark tongue spots can be found in many different breeds. It´s noticeable that these are mostly breeds where dark pigmented mouth mucosa (lips, gums, palate) is preferred (this is an example for the FCI-standard of the Spanish breed Gos d´Atura: “lips: (…) strongly pigmented with black, the palate is also black”.
Such a selection obviously leads dark pigmented spots on the tongue. There are no known health related problems.