To my knowledge, CBDCs will not be using blockchain
I agree that first proposals around 2015-2016 (e.g. by the Bank of England) mentioned blockchain, up to a point of actually suggesting to use it as their backbone value transfer system. CBDCs were in fact inspired by Bitcoin and its technology. However, from what we now know, the real implementations won't be based on blockchain. Seriously, what's in it for central bankers? In my opinion, they would do just fine with centralized solutions
A CBDC will prove to be a safe-haven for banks worldwide, as they'll gain more control over people's money. Privacy will be a thing of the past, as banks alongside governments, will control the centralized Blockchain ledger allowing them to do as they please. With no physical cash, every transaction performed will be entirely visible to said entities. I think this is one of the many reasons why central banks and governments have become so interested in a launching their own CBDCs. It seems that China will be at the forefront of the digital economy, as it's already massively testing its own digital version of the Chinese Yuan. Other countries might start doing the same soon, in order to avoid getting left out of the game.
Once the whole world relies on CBDCs for mainstream transactions, then the battle between the centralized and decentralized realm will begin. While I don't think decentralized cryptocurrencies like Bitcoin and Ethereum are going anywhere, they'll be challenged by CBDCs worldwide. After all, CBDCs will provide virtually the same benefits of crypto (speed and low costs) while retaining the same properties of physical cash (especially price stability). It'll be up to the people to choose from one type of currency from another. While CBDCs will increase government surveillance, people who want privacy over their transactions will rely on decentralized cryptocurrencies we all know and love today. Using Blockchain tech on a CBDC makes no sense if the same will be utterly centralized. It'll be nothing more than a "glorified database" subject to manipulation and corruption by the world's most powerful governments and central banks. Just my thoughts