A blue pitbull is an American pitbull terrier with a blue-colored coat. The color can range in hue from a light gray-blue shade to a dark charcoal color. The nose is gray in color as well.
The blue color of the coat is a recessive genetic trait called a "dilution" of the color black. The genetic trait causes the black pigment to clump in the middle of each hair, leaving the tip colorless. This effect causes the dog to appear blue to human eyes. Because the trait is recessive, for a dog to have a blue coat, both parents must have a blue coat.