The successors to the Blackbird are satellites dude.
Why fly over the area you want to monitor when you can continuously monitor it from various satellites.
The Blackbird goes so goddamn fast so it doesn't get shot down like the U2 (Did we forget about Gary Powers already?)
Not entirely...
There are problems with using satellite surveillance.
Starting with: you can't really "hide" a satellite. Stealth dosen't work for something that you, yourself, need to see in order to talk to in order to command. Also, solar panels are reflective; so-much-so, that they are naked-eye visible from the ground at certain times of day. It's impossible to have 100% coverage, as the price approaches infinite (the CIA's black budget is a LOT, but it isn't infinite), you can go higher for better coverage, but there's a wicked trade-off in the massive loss of resolution, so if you want detail, you have to go low. Combine all of these facts with the fact that most spy satellites are in LEO with regular, predictable orbits, makes it relatively easy for guys on the ground to play hide-and-seek during flyovers.
There was a funny story I read in a
book once, about the guys who worked at Groom Lake during the 60's, 70's, and 80's. They would play jokes on the Soviet infrared-reading spy satellites.
The crews working there had to be
very aware of the Soviet flyover schedule because they had to make sure the sensitive stuff (in the book, not-so-legally-aquired MiGs, although there was
other stuff too...) had to be put under cover. They also had to be aware of the
shadows cast by the planes in the hot Nevada sun, which the infrared-reading satellites could see as silhouette left on the tarmac. Well, someone got the idea to
deliberately cast fake shadows right before a flyover, so as to, as the Brit say; "take the piss."
Two standouts mentioned: were a 1/1 scale NCC-1701 and the Batwing from the 1989 Batman movie.
There is still a need for spy planes see where the satellites can't.