Why did medusa used black magic to form or rearrange her face as she did not want anyone to see her beauty anymore?

Yeah, I don't know where you are getting your information but stop now, it's from a bad source.

Medusa of Greek Myth is called the daughter of Phorkys, this being the god of the dangers of the sea, Medusa as his offspring is a monster born of monstrous gods, she was always a monster, lovely or not, and always turned living things - and people - to stone, she and her sisters were a explanation of reefs and how ships were caught by them.

Athena, as a goddess who cursed Medusa for coupling with Poseidon in her temple - and made her "so hideous as to turn men to stone", is another aspect of Greek myth to explain why Athena - a virgin goddess - helped Perseus.

No where in the Greek myth surrounding Medusa do they use "black magic to form or rearrange her face" because Medusa is somehow ashamed of her beauty or wishes to keep it for herself or not be seen by men.