If you were a bank or a set of banks and just "don't want anybody to be able to modify the ledger's history" and you know what you are doing, you actually don't

If you were a bank or a set of banks and just "don't want anybody to be able to modify the ledger's history" and you know what you are doing, you actually don't want to use blockchain. Because blockchain doesn't mean "nobody can modify the the ledger's history". It means "for it we have to use "proof of work"". But the "proof of work" means "whoever has 51% of the computing power can take over all". Of course, blockchain promoters would tell you that "there will be something else, comes Really Soon (tm) which won't use "proof of work"." One can more efficiently use public key cryptography, hashing and signing without the blockchain for any other problem than "implementing bitcoin."