No, I'm pretty sure it's possible in a lot of breeds, genetics mostly. I know a golden who has just enough extra pigmentation to make his tongue look purple! (it's not lack of circulation or anything, the owners had it checked out)
This is a myth that started because it is VERY common for a chow to have a black tongue. The popular Lab can have black spots as can German Shepherds and a variety of other breeds. My dog has black spots on the underside of her tounge and is a boxer mix (most likely lab).