I'm thinking in the long long term of hundreds of years. Seems to me when we start exploring space moving around heavy chunks of metal won't be so useful anymore and we'll need to use a Blockchain to verify value transfer.
While I agree we will probably want something different from the current status-quo when light-lag gets big enough to matter for normal daily financial interaction, one potential solution could be to go back to local banks holding money locally, just as things were here on Earth before ATMs and cheap telephony.
We haven't been using heavy chunks of metal as currency for a while without a need for blockchains. I see mostly disadvantages compared to government backed currencies.