Blue is an exotic coloration of the American bulldog, though not regarded as a standard or non-standard shade by the American Kennel Club. A blue American bulldog starts as black. The color dilutes to blue eventually as it reaches about six months of age, mainly due to genetic reasons. Because of the dilution, the blue coat of these dogs could have a tint of gray or even appear bluish-gray.
Blue Merle
For the blue merle American bulldog, a portion of its black coat remains intact, while some of it dilutes, attaining a bluish-gray coloration. The blend of both results in a merle pattern seen throughout its body as dark patches.
