A bluish or purplish tinge to your dog's tongue is generally not a cause for concern. Dogs with black or pigmented spots (such as chow chows) may naturally have blue-black spots on the skin of their tongue.
This is simply caused by the melanin pigment that gives a dog its color, and it is completely normal. However, if you notice that the spots are changing color, shape, or size, it is best to have them checked out by a veterinarian just in case.