Dear Student ,
Black holes does not violate the law of gravitation in space because they have strong gravity effect that no particle and electromagnetic ray such as light cannot escape from it . The centre of a black hole is called "singularity" where the gravity is very high and mass of a minor black hole is almost equal to the moon and of size 0.1mm . Now when in such a small size a lot mass can stay then how black hole can violate law of gravity . The main thing is that only "singularity" is the main thing which proves that a black hole violate gravity.
Regards