That is really good news, although I find it quite surprising. I know of no accounts of black amur leopards in the wild and their habitat is not the kind conducive to melanism. (In cats, melanism is almost exclusively confined to warm tropical forests, the only exception I know of being the high alpine moorland of The Aberdares, Kenya).
I'm no expert on how leopards evolved/expanded geographically but maybe the Amur Leopards are more closely related to Asian leopards which I understand are more likely to be melanistic? Additionally maybe there's never been a black Amur Leopard spotted in the wild as their colouration means they struggle to survive in their habitat?