My understanding is that blockchain isn't really useful "behind the scenes." It's value is that it can be exposed to the public without being messed with or traced. Banks want everything to be traceable and secret, so they are better off keeping private ledgers. The only reason it is appealing to the mob is because they have to worry about getting exposed, so you can't have a private ledger with a list of all your crimes in a safe in your office the way a bank can.