Central banks operate provide real-time gross settlement (RTGS) systems. This allows transfers to settle in real time.

See https://en.wikipedia.org/wiki/Real-time_gross_settlement

Domestic banks have accounts with the central bank and can move money using RTGS. If the central banks allowed individuals and non-bank companies to open accounts then they could use the RTGS to transfer money instantly.

There would be no blockchain, the central bank acts as a trusted third party. The central bank can see all transactions.