Dog's gums can turn black when a dog has dental disease, or they can simply naturally be black according to the American Kennel Club and the Massachusetts Society for the Prevention of Cruelty to Animals. If a dog's gums have a bluish or gray tint to them in addition to the black then the owner should bring the dog in to an emergency veterinary clinic immediately because bluish or gray gums are a sign of a cardiac emergency, says the Veterinary Specialty Hospital.
If the dog has a bluish or gray tint to the gums, it will also likely have a rapid or slow heart rate and may be showing gasping breathing signs says the Veterinary Specialty Hospital.
More commonly, black gums are a natural part of the dog says the American Kennel Club. Dogs have gums that match the color of their skin beneath their coat and many dogs have a mixture of pink and black gums. The black may be visible in spots, patches or may be the only color visible. It is recommended that dog owners take a look at their dog's gum before an emergency or crisis so that they know what their dog's natural gum color is.
Another possibility for black gums is dental disease says the Massachusetts Society for the Prevention of Cruelty to Animals. Dogs with dental disease often have black or brown tartar on their teeth and smelly breath. These dogs will need dental cleanings or they run the risk of developing multiple health problems.