Getting blackmailed for essay

I registered for a website that helps with essay writing. It said to pay $1 to register which I did. Hours later, my credit card was charged and a person messaged me on Whatsapp with rude messages when I asked why I was charged. I had since made some contact with them on Whatsapp. I have since blocked them. They are now blackmailing me that they will message my university. I called my bank to dispute the transaction. I received something from them but it was a random case; they don't know which specific case even. I also marked that email as spam. Should I be worried?

You have nothing to worry about (other than identity theft from the info you provided). Follow up with your credit card to dispute the charge and they'll probably close the card and issue a new one if you tell them what happened. Block every means by which they contact you and don't respond to anything from them, at all. Read all the other threads dealing with similar problems on the main page.

You have nothing to worry about. They do not have any information that they can use to academically blackmail you. It is good that you did not progress in dealing with them and that you cut them off from contacting you. I would be more worried, as @freelancewriter said, with identity theft more than anything else. That is what they may resort to should they fail to scare you with academic threats. How long has it been since they last made contact with you? Make sure to keep track of your bank balances and card charges in case they do try to suck money out of your account. That is the only path left for them to follow.