I'm no fan blade expert. I have only ever done this to motors not connected to fans.
But unless I'm mistaking (which is a VERY rare occurrence), reversing the direction of the motor will result in the airflow direction being reversed as well. Just as is the case with propeller on a boat.
Here is a fun experiment for the whole family (not really). Get a fan. Hold it so you are facing the "pretty side" (airflow away from you). Now blow into the fan. It should turn in it's normal direction (as indicated by arrow on fan frame).
Now turn the fan around and repeat the process. Notice how the blades spin in the direction opposite what is indicated on the frame.
In that "experiment" your breath played the part of the fan's motor. See how that works?
Considering the design of most PC fan blades, I highly doubt that it would be anywhere near as effective in reverse. But that is naturally down to each individual model of fan.