Most people have heard of Albinism which is the congenital disorder that is known by the complete or partial absence of any skin pigments. The pigments are often lacking in the skin, hair, and eyes. Normally this results in white hair and pink eyes when it comes to mammals. When it comes to animals it can be very deadly as they have no protection to UV light or any form of camouflage to avoid deadly predators.
What is less known and spoken about, due to it only existing in the Animal kingdom and not humans is Melanism. This is when the animal is completely black. The development of the dark-colored pigment melanin in the skin. This can be caused due to a genetic disorder but it is in most cases actually naturally evolved due to adaptation.
For example, the black panther is completely black because his genes adapted to become a better form of survival in their natural habitat and environment. They became fitter to survive longer and melanism enabled them to be better camouflaged.
Adaptive melanism is usually heritable and happens in species like squirrels and many felines. However, sometimes melanism is directly caused due to human pollution. One moth is known to have become completely black solely due to the amount of pollution in the air, meaning by being black the peppered moth was better camouflage.
In humans, the mutation that causes completely dark skin does not actually exist. Remember that melanin is essential to human life allowing us to create vitamin D and live healthy lives. In fact, the amount of melanin in a human has adapted related to the further away they traveled from the equator. This is due to melanin protecting our skin from harmful sunlight, so the further from the equator you live the less you need means you are naturally lighter skinned.