Is it a possibility there was impurity in the bloodline?
I don't know if these cubs are related to the Black Amur Leopard they have(had?) for some time? I believe there has always been a question mark over that animal's purity(it was born at Cotswold) as it was thought Black Amur Leopards didn't occur in the wild..
I think since then a black morph has been recorded in the wild Amur leopard population- though that isn't proof of the Thrigby animal's purity of course.