There is some controversy over just how that happens. If gravity is made up of particles, it should not escape. But they say that a static gravitational field is made up of virtual particles that can travel faster than the speed of light.

It is known from experiments that EM fields that are critically damped- that have exponential dependence on distance rather than being wave like- travel faster than the speed of light.

Anyway, we only know about black holes from the motion they impart to stars outside the black hole that we can see. We can measure the mass of a black hole from the angular momentum of the stars.