Chagrin:
A brushless fan as found in a computer contains the controller within the fan and that controller is only designed to spin the fan in one direction. Fundamentally though, brushless DC motors are certainly reversible.FWIW there are zillions of RC cars with brushless motors and reverse capability.
Thanks- what what I expected. Or, at least I would have been surprised if they couldn't run both ways, and thought there might be something in the motor preventing it. I wonder why though?- the plug is made to fit one way only, and since the fan is hidden inside it's not likely to get connected back to front in use.