YF12A Blackbird mission capabilities?

I’ve long harboured a niggling memory of the interceptor variant of the SR71.

I seem to remember that the programme was shut down because, allegedly, of cost. On the other hand, like most people at the time I could not understand what this machine was actually supposed to do.

Most of its mission profiles were actually spoken for by slower birds of the time which were a good deal easier (and, yes, cheaper) to build.

Was this a case of a designer’s being too clever by half or were Lockheed genuinely hoping for a large contract? The whole thing seems cockeyed.