umm before the union,,,the first part of the pipe to come outside (because you want all the water to be out) you can install a Tee with threads to hook up the compressor to, and then you can blow out before the Backflow and you do not have to bring it in for the winter. You can buy an adapter depending on what kind of compressor you have at the local hardware store, because i dont think sprinkler warehouse has them....let me know!
Thanks
Ray
Rays Above and Beyond Automatic Lawn Sprinklers