Hey, they just announced this- the Darkstar is real! This is a super secret US Government mach 6 spyplane, they just went public with this! However I would bet anything they already have something even more advanced than this, the reason why they went public was that they already have something better! They have pictures of this thing on the internet.
Here Are The First Images Of Lockheed Darkstar On Display At Edwards AFB
Supersonic / hypersonic planes makes terrible spy planes with their massive infra-red signature that can be detected thousands of miles away.
A good spy plane should not only be difficult / very hard to shoot down but also be invisible to observation / detection.
Because even if say Russia can't shoot down an American spy plane, having an American spy plane over-fly Russian air space gets detected, caught on scanners, could give Russia the justification to conduct hostile actions against USA, for example conduct spy plane flights of its own over USA or even engage in cyber-espionage, etc.
Ofc, USA can always make up an excuse like "navigation error" why its spy plane ends up going over Moscow for example.
If Russia retaliates with similar spy flights, they can also make the same excuses so the best option is never get caught. A spy plane that is virtually invisible from detection. Would certainly be subsonic and would employ the latest in stealth tech.
Thus, I can only think that these hypersonic spy plane concepts are just "decoy designs" where the real work being made with a completely different plane, mostly likely a subsonic plane. Best bet is making it unmanned / autonomous to minimize weight and size and also to maximize operating altitude.
I've wondered if fully "radio transparent" stealth design is possible. It could be technically difficult but such stealth would be invisible to all radio frequencies. The current method of stealth - partial radio absorption and reflection can still be detected by high power, low frequency radars.