The Blues routine involves a failry complex manoeuvre by the 4 ship which gets the crowds attention the the 2 solo jets sneak in from 2 different directions one after the other. They are very low and very fast and you're right to say that you don't hear them until they are past you.
However, they aren't that far away, as you can see from this shot which I took at their homecoming show this year. Having seen the show before I knew where to look for one of the fast pass aircraft which came straight along the crowd line, the other came from behind the crowd line about 2 seconds after this one had passed. I'd estimate him to be well below 100'
Mac
