Yep, you blew it. That was an infield fly, since it was a fly ball that could have been caught with ordinary effort by an infielder. How much simpler does it have to be?
No umpire has to call this. The batter is out whether or not either umpire says a word, and whether or not anyone hears them. It's a live ball, so the runners are entitled to adavance at their own risk.
I had one like this on Thursday. It was a windy day, and the D-coach didn't hear us call infield fly. The ball dropped to the ground next to F5, and the runners began to advance. F5 picked up the ball and ran over to step on 3B, but since the batter was out it was not a force play.
The coach complained that I hadn't said anything. I told him that I had (and his catcher confirmed that), but that the batter's out anyway on an infield fly. Naturally his team hit an infield fly later in the game, and he heard me call that one.
