Black Hat hackers, also known as crackers, are those who hack in order to gain unauthorized access to a system and harm its operations or steal sensitive information.
Black Hat hacking is always illegal because of its bad intent which includes stealing corporate data, violating privacy, damaging the system, blocking network communication, etc.
So, i think you understand very well what is Black Hat Hacking. In Easy word if someone access the data of another person without his permission. Then that is known as hacking or you may call back hat hacking.
If you have some questions about this then leave comments i am try to give answer soon as soon.