A "blue" cat is actually a type of grey coloration in cats, sometimes referred to as "blue-grey". The blue-grey coloration in cats is a result of a dilution gene, known as the dilute gene, which affects the distribution of pigment in the hair shafts of the fur.  RELATED:  Why are gray cats called blue? Extreme bred blue British Shorthair probably Russian bred. Photo in public domain The dilute gene causes a reduction in the amount of pigment, particularly black pigment, in the hair shafts of the fur. This results in a dilution of the black coloration to a blue-grey color, while the other color pigments (such as red and yellow) remain unchanged.  RELATED:  ‘Blue’ is the dilution of a black cat coat . Blue-grey cats can come in a variety of breeds, including the Russian Blue, British Shorthair, and Chartreux. It's important to note that the blue-grey coloration is purely cosmetic and does not affect the cat's behavior or health in any way.