For these kinds of radars, there won't really be any signal against the sky and, lets be honest here, nothing is going to be looking at the SR-71 from a top dow

For these kinds of radars, there won't really be any signal against the sky and, lets be honest here, nothing is going to be looking at the SR-71 from a top down aspect. In a tail chase situation though, a missile would have to fly straight at the exhaust which would give the aircraft some cover. There's a bit of reading between the lines that needs to be done here though. The aircraft is already shaped to deflect radar waves from a tail or nose on aspect but the part that's very hard to 'stealth' is the insides of the engines, especially from the rear aspect. Turbine blades and all the other geometry inside tends to act like a radar disco-ball and leaves even many modern aircraft vulnerable. By making the exhaust radar absorbent you fix this issue.