M2dude
I know this is the CONCORDE thread, and Blackbird was a twin that operated at 3+, so I'll not stay long, but only share what her pilots have shared with me.
Bird burns a one-off fuel, I think J-8. It will not light on start up without a hypergolic chemical reaction with injected ethyl Borate. I am unfamiliar with airstart, but believe it to be impossible following an unstart. If one engine unstarts, the Yaw is nearly 90 degrees, and instantaneous. Rather than wanting to be in dense air, the only hope is to be at thin air, and hope the a/c does not tumble. I have heard estimates that after unstart, the event is 50% fatal. There is a "club" where one's trophy is the helmet, worn at loss of sym thrust. The helmet strikes the side glass and folds in two. I think it is the "crease club".
Her pilots, (there are but two seats) are well trained for all possibilities, and have an edge over CONCORDE in that they have weighed the odds. The commercial ship is engineered to closer risk taking, and her limits also dance at the "edge".
One last SR-71's Size. Don't get the impression she is dainty, she has the shadow of a 727 classic. The Blackbird may have wider limits, but the Concorde put money in the Bank. The Blackbird "stole" two or three Banks per mission.