Money as a concept is even less anchored in the real world. So is the law. The two have been dancing together since first human society. We have thousands of years of experience with this, and cryptocurrencies don't introduce anything new here, despite what some advocates may think.
As long as you can use something - be it a coin, a bill, a SWIFT transfer, a blockchain transaction or a Nuka-Cola bottle cap - to pay someone for something, and a third party would be able to identify it as a purchase, same things will happen. A criminal will be able to take your purchasing power and use it themselves. A taxman will be able to collect a percentage of your trade, and men with guns will take you in front of a judge if you refuse.
The manifestation of value doesn't matter, because money isn't a thing, it's a shared belief.
While I agree, all of the ways you mentioned to pay for something are fundamentally something that requires infrastructure to create (industrial, banking, or a mint), and has a physical anchor that can be plucked from you by physical force (including the SWIFT transfer - though usually bank's prefer to comply than shut themselves down).
A blockchain based currency will exist as long as there's someone with a sufficiently powerful computing device connected to a sufficiently low-risk network that can be accessed legally or illicitly. A blockchain wallet will exist and be yours alone as long as you keep the password in your brain, never write it down, and resist attempts to extract it from you.
This means that, in comparison to conventional currency (including Nuka-Cola caps), a blockchain economy is much easier to operate undercover when it's outlawed.
As long as you can use something - be it a coin, a bill, a SWIFT transfer, a blockchain transaction or a Nuka-Cola bottle cap - to pay someone for something, and a third party would be able to identify it as a purchase, same things will happen. A criminal will be able to take your purchasing power and use it themselves. A taxman will be able to collect a percentage of your trade, and men with guns will take you in front of a judge if you refuse.
The manifestation of value doesn't matter, because money isn't a thing, it's a shared belief.