They’ll use the same techniques they are now: monitoring and blocking cash flows, getting partners not to use it, etc. This is the same way that drug dealers get caught even if they try using a variety of currencies, collectibles, real estate, etc. — at some point you need real currency to spend and the in/out-flows are monitored.

All a cryptocurrency adds is a public record making that task easier and the hokum used to sell it may make the NSA’s job easier if people believe it’s anonymous. Setting up a separate one also means there’s less benign traffic to hide behind.