First you have to define mass. It's currently "defined" extrinsically, by a piece of metal machined by the SI. That doesn't allow an intrinsic answer definition

First you have to define mass. It's currently "defined" extrinsically, by a piece of metal machined by the SI. That doesn't allow an intrinsic answer definition of a black holes mass. The answer is kinda easy if I can make up my own intrinsic definition. The mass is the mass of the stuff around the black hole. A black hole is a singular point, it can't have mass, don't be silly.