In 2017, Jamie Dimon, JPMorgan Chase’s chief executive, declared Bitcoin a “fraud” and said that any employee caught trading it would be fired for being “stupid.” On Thursday, JPMorgan became the first major United States bank to introduce its own digital token for real-world use, the latest step in Wall Street’s evolving approach to the blockchain technology that underpins cryptocurrencies like Bitcoin and Ether. Despite questioning Bitcoin’s legitimacy, Mr. Dimon has said he recognizes...