Don't stay alone in this situation ! this bully need some lesson .
Because there is no strait answer for this , my best advice to you is to get help from adult that know you best and you are trust on.
Don't give up to blackmail ever , get close help - believe me that cost you less shame from take control by your self.
Blackmail in most countries consider as crime - the Police is also good solution but also there is more recommended to be with adult you trust.
My inbox is always open feel free to write .