Does black holes violate the second law of thermodynamics?
No it does not! Black holes has the most entropy in the universe.
Where does all that entropy go?
Most of the entropy lost by the materials that fall inside a black hole is rescued back to the universe by Hawking’s radiation. Hawking (with Bekenstein) also proved that the surface area of the event horizon is proportional to the mass of the black hole, thus proving that the entropy is preserved in the universe.