Using the blockchain, a technology that's shown to have trouble handling single digit numbers of transactions per second, to scale for billion of accounts? That's interesting.
I wish people in this entire thread weren't allergic to providing citations and references though.
There's nothing about blockchain's that prevents them from scaling beyond single digit transactions per second - you're referring to the design decision Bitcoin has made to opt for a smaller blockchain in order to ensure distribution and decentralization.
A settlement blockchain between financial organizations doesn't have the same design parameters, and could conceivably run larger blocks, shorter block times, or not store block history at all but rather reach consensus on state (which is what Ripple does)
Cryptocurrency uses blockchain but blockchain isn't cryptocurrency.
Blockchain technology is already incredibly useful for banks doing interbank currency trading iirc.
I guess you could be talking about banks doing crypto trading, then I would guess that they are but a number are still stuck in the stone ages.