The Russians apparently fessed up after the cold war that they knew of the SR-71 project from early on, but the only thing they knew about it in the early stages was that beautiful and distinctive outline.

The Skunk Works team knew when Russian satellites were coming up over the horizon, and so would hide the early development models they had on poles and so on. They obviously didn't want them to be pictured. However the satellites had heat-sensing cameras and so could see the heat differential the model had left as a shadow on the ground just minutes before. The Russians therefore knew there was a new plane in development, and they knew what the outline of it looked like, but nothing else for quite some time.

I bet when they first saw it, they agreed - a heck of a beautiful machine, one of my favourite aircraft of all time.