Countries without stable currencies tend to use things like USD which have stable values rather than wild fluctuations. That’s a product of a social system and years of practice, not hand waving about magic real-soon-now technology. In your Russian example, the value would be in the link to the commodity rather than whether it’s a printed bill, entry in a database, or record in a block. If that’s honest, any of them will work and if it’s not, none of them will.

Besides, if I lived in Venezuela or Russia, would I really want to record my black market activities in a public ledger which is hard to disavow?