Blue is a color description used to describe a silver-blue color that some animals have. This color is sometimes refered to as grey or charcoal, as well.
There are a few cat breeds that are blue, such as the Russian Blue cat, the British Blue cat, and the French Chartreux.
Blue cats can also be found in the Persian cat breed and even the Spynx cat. Some calico cats feature blue markings, as well.