Oh, it is. Blackmail is the threat of releasing information, whereas demanding payment to stop publishing information is extortion. I think a lot of states treat them as legally similar crimes though.
I’m skeptical that many drivers will pay to have their plates de-listed. Extortion schemes like mugshot sites work because they attach identifying information to something for which the stick-up-their-ass crowd will discriminate against people, namely having been arrested (whether they were charged or not). But the only way this extortionist will be able to obtain identifying information is if and when someone pays the extorted money non-anonymously.
It’s illegal for non law enforcement to run plates and vehicle registration databases are not publicly available. In the unlikely event that the extortionist is himself LE, he’ll be discovered quickly if he tries to run a lot of plates in a short period of time.