Secrets are hard to keep.
Key point: Black projects can be kept in the dark, but not forever. Here is how folks eventually found out about the existence of the SR-71 blackbird.
There are few secret projects like U.S. Air Force black projects, and that was certainly the case for the famed SR-71 Blackbird.
The SR-71 was no ordinary aircraft but a big, beautiful and state-of-the-art spy plane designed in the 1950s and early 1960s to fly quickly at high altitudes over the Soviet Union, filling in for the U-2 Dragon Lady which had become vulnerable to then-new surface-to-air missiles.