Look! An all-black King penguin! How odd!
His picture was first captured by Evan Andrews, a National Geographics photographer. He's actually extremely rare. So rare, that there isn't any research on what sort of genetic mutation might cause this. Doesn't seem detrimental to the animal though - there appears to be no differences when compared to the normal white penguins.
The black penguin has melanism - the opposite of albinism. In melanism, there is the presence of melanin (the dark pigment) where it shouldn't be (whereas in albinism, melanin is missing). Melanism is common enough in birds & other animals, but usually this increased pigmentation is patchy - often in stripes or spots. Melanism is NOT the natural coat of the animal though - take a dalmatian for example. They are not naturally melanistic (even though they have black spots) but if you found a mostly black dalmatian with only a few white spots, then that would be melanism. What's rare about this penguin is that ALL the white feathers are black - no stippling or salt & pepper to it at all. Which means ALL of the feathers had to gain the ability to produce melanin. A rare feat indeed.
What surprised me was the common example of melanism - the black squirrel! Apparently, they're just a (melanistic) subgroup of the Eastern gray squirrel! Here you can see a photo of a black squirrel with incomplete melanism (some gray patches).
Melanism is often an adaptation - black animals might have an advantage when it comes to hiding or to hunting in the night.