Psittacine Beak and Feather Disease (PBFD) is the probably cause. It is incurable and it is very uncommon that black cockatoos of either genus can get the virus, it is usualy seen commonly in white Cacatua cockatoos.

PBFD is generally considered fatal, with most infected birds surviving 6 months to 2 years after the onset of clinical signs. The PBFD virus is immunosuppressive and death usually occurs from complications due to secondary bacterial, viral, or fungal infections, or from terminal disease changes which necessitate euthanasia.

You may want to obtain a biological sample to send to a diagnostic lab for confirmation...