I talked to Lockheed engineers (at a party) thirty years ago about the supposed successor to the SR-71 which was all the rage back then: the Aurora. Obviously they didn't tell me anything but it was nice joking about it.

It struck me they were trained to respond to questions about classified stuff. When you asked them a question they responded with a question.

So yes, most probably something has been flying for decades that's at least as fast as the Blackbird. Prototypes which may or may not have been put into production.