A blue merle pit bull is a cross-breed of an American pit bull, or Staffordshire terrier, and a catahoula leopard dog. It is the only disqualifying pit bull coat color combination for inclusion in an American Kennel Club competition due to its mixed bloodline.
Blue merle is a pattern of a dog's coat, not an actual color. Blue merle patterns include a solid base color with blue and white or blue and black patches. This pattern can also change the dog's original dark eye color to blue or cause portions of one or both of his eyes to turn blue. The merle pattern is also responsible for causing a dog's nose and paw pads to have pink and black specks.
Blue merle pit bulls are largely found in the South, where there is a large concentration of both catahoula leopard dogs and pit bulls. Although the AKC, the United Kennel Club and the American Dog Breeder's Assocation do not officially register blue merle pit bulls, the American Pit Bull Registry issues certificates for all blue merle pit bulls that are proven to be full-blooded.
Other dogs that can also have blue merle markings include Old English sheep dogs, Pyrenean shepherds, Welsh corgis, Koolies, Australian shepherds, German coolies, Pomeranians and Chihuahuas.