There are two distinct classes of system pushed under the label “blockchain”: the kind like Bitcoin are enormously complicated by trying to be used between untrusted parties. The systems which are potential viable are much less expensive because all of the parties in their system are known and trusted at least to be subject to contract law. All a bank needs is public key encryption and Merkle trees, and while that might be branded as a blockchain for marketing purposes that doesn’t make it the same.

Or they could just start doing real time settlement, something which the banks are working on.