Black tongue in cats can happen with injury, but it is more likely a result of an immune dysfunction which can be caused by yeast overgrowth or more serious infections. The literature does suggest the direct cause of black tongue by a specific agent like an injury or burn; anecdotal evidence suggests that black tongue most likely is the result of a toxoplasmosis infection due to parasitical infection.
Eliminate burn sources
Eliminate burns or caustic materials as a possible cause of your pet's black tongue. Are there open containers near your pet's environment? Are there pills missing or spilled powders nearby? If so, take your pet to the veterinarian as soon as possible. Examine your pet's area for frayed electrical cords. An open light source near your pet's area might indicate a tongue burn.
If your cat has a yeast infection, it may result in a black tongue. Yeast infection may be the result of an immune dysfunction whose underlying cause might be stress or a viral infection that has caught hold. Although the majority of cats are infected with feline herpes virus, most have no symptoms. Something that disrupts normal immune balance of your cat can lead to a yeast infection. The virus disrupts normal cellular function, allowing yeast infections to flourish, thereby causing black tongue. Nasal swelling and discharge in your cat may be signs that feline herpes virus-1 is expressing itself in your cat.
If your cat indeed a has a black tongue from an overgrowth of yeast, it might be advisable to add a small bit of plain yogurt to your cat's food until the black tongue clears up. The overgrowth of yeast as well as the immune dysfunction may be the result of poorly balanced bacteria in the cat's gut, and the yogurt can help the cat re-balance the gut bacteria. Your vet might also prescribe miconazole for your cat's yeast infection.
An infection called toxoplasmosis is caused by a parasite which may be the root cause of your cat's black tongue. Toxoplasmosis may be obtained from infected food, but can also be passed from animal to animal or human through contaminated excrement. Do not feed your cat raw meat. Additionally, make sure to clean your animal's litter box every day, and use gloves and a mask when you dispose of the feces. Do not let your cat eliminate waste in your garden, where it may share its infection with your vegetables.