All-black penguins are so rare there is practically no research on the subject–biologists guess that perhaps one in every quarter million of penguins shows evidence of at least partial melanism, whereas the penguin we saw appears to be almost entirely (if not entirely) melanistic….

Observing this black penguin waddle across South Georgia’s black sand beach revealed no different behavior than that of his fellow penguins. In fact, he seemed to mix well. Regarding feeding and mating behavior there is no real way to tell, but I do know that we were all fascinated by his presence and wished him the best for the coming winter season.

Lots of discussion about this photo and this kind of coloring, plus other links, at the National Geographic blog entry about it.