This question is sparked with an answer to another one. We know that fields have mass-energy and gravitational field is no exception, some share of BH mass should be contributed by its gravitational field.
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The energy in a gravitational field cannot be localized in GR. The Newtonian expression for the energy density of the gravitational field is proportional to $g^2$, where $g$ is the gravitational field, but the equivalence principle tell us that $g$ is not observable in GR. For example, we can make $g=0$ at any point we like, simply by adopting an inertial frame of reference. There is a detailed discussion of this sort of thing in Wald, section 11.2. When we talk about the mass of a black hole, we're talking about some quantity such as its ADM mass, which is essentially the mass measured by a distant observer.