Extortion could be as minor as demanding a discounted rate in return for either writing a good review, or NOT writing a bad one. Or it could be as serious as someone threatening to "out you" or worse, demanding payment for his/her silence.

Some extortion attempts warrant no more response on your part than simply not responding at all, as Melissa suggested, and of course her further advice to keep "EVERYTHING" where it comes to emails, texts etc. OTOH some blackmail is serious enough to get LE involved even though you are involved in an illegal business.  

My advice to do as Melissa advised and simply ignore this person if possible, block them from both your phone, email and TER if applicable. Now if this is something that rises to the level where you can not ignore them and they have a virtual gun pointed at your head, getting LE involved might be necessary. If you do decide to get LE involved, PLEASE do not do so on your own, as you might find yourself the one behind bars. You really need to consult with a lawyer and let the lawyer deal with LE for you, he will know what he can and can not disclose in order to make sure you don't become the target for LE instead of the blackmailer.

Without knowing what kind of blackmail you are dealing with, it's impossible to give any more specific advice. If you feel like sharing some details, I am sure you can get some more detailed advice, but keep in mind, advice is almost always worth what you pay for it. and that goes double for "free" advice you get on the internet, but we will do our best.