Despite the name, however, J-Coin has even less to do with cryptocurrency than the recently announced JPMCoin, which in cryptocurrency terms was dismissed by many as a pretender.
The wallet app, which became available to Japanese citizens on March 1st, is the result of the collaboration of sixty banks, and represents a way to complete all payment-related processes, including sending, receiving, and paying money, from a single dashboard.
https://paperblockchain.com/japans-banks-launch-digital-coin-but-blockchain-is-nowhere-to-be-seen/
As long it is centralized there is no reason to set it up in a chain of blocks. It would be dump if they do so.
In most of the cases, when the word "blockchain" is used is in order to create hype and make people fomo. The Japanese government shows integrity of creating this ecosystem in order to help their citizens and not making profit on that.