Pedantry: the aircraft at the Museum of Flight is actually an M-21 blackbird (with a D-21 drone attached), not an SR-71 blackbird. It's the only surviving M-21.
There are about 20 SR-71s in museums (plus an SR-71 cockpit at the Museum of Flight, which you can sit in.)
There are about 20 SR-71s in museums (plus an SR-71 cockpit at the Museum of Flight, which you can sit in.)
Sources: http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Accide... plus personal knowledge as a former Museum of Flight employee