yup! you are right i read the part again after writing the previous reply. The x-rays are not emmited by the black hole itself but the matter at the event horizon. This is the reason why x-rays aren't absorbed by the black hole
i was reading a brief history of time and i answers very clearly the very question you asked. Though black holes absorb all light(one way to detect anything) it does have a observable gravitational effect through which the have been proved to exist. they also release x-rays which they don't suck...
the mathematical equations that predicted the existence of black holes have another solution which predicts white holes. but it has been proved that white holes cannot exist. it was thought that they exist in a setup called wormholes in which they are other ends of black holes but this setup was...