I have a friend who is being threatened by anonymous accounts threatening blackmail. I have gotten multiple ip addresses from an iPhone, all 1 number off, from this person but they were stupid enough to tell me the vpn they use (widscribe).
I would recommend contacting local police and reporting that incident as well as contacting other international authorities. Usually, such threats are being fought by special cyber units.
I would not recommend contacting the blackmailers or reacting to their provocations. Just compose a descriptive detailed report and send it to the police.
Sometimes criminals are waiting for the victim to make such mistakes to exploit those mistakes against them, so be careful with contacting them or threatening them.