Now, have they? I was surprised to hear this because I was under the impression that blockchain is generally useless in an environment where the participants can trust each other. I searched for "visa jp morgan blockchain" and found these:
https://www.forbes.com/sites/emilymason/2022/10/11/jp-morgan...
https://www.finextra.com/newsarticle/41102/jpmorgan-and-visa...
It seems that JP Morgan is offering a service called "Confirm" to validate account numbers, which Visa is using. This service is apparently offered on a blockchain. But nowhere does it say that these companies would be using a blockchain to actually settle real transactions. And it indeed seems a service to validate account numbers could just as well be offered with a more traditional API, like HTTP/JSON, instead of blockchain.