Lawn Mower Blade Balance Issue?

I just got a new lawn mower last night. I mowed the lawn today and never thought to raise the blade up from the lowest position before using it. I ended up hitting a big clod of dirt (from a mole I think) and it stopped the blade cold. I finished the lawn, but the mower vibrated more than it had been before. I pulled the blade off and got rid of the dents in the blade with a file. The blade is very unbalanced though. It isn't bent, but in order to balance it you would need to grind off about 1/4 inch of material on one edge. I know hitting the dirt didn't cause enough damage to make the blade that unbalanced. The "driveshaft" or whatever you'd call it on a lawnmower looks fine and isn't noticeably bent. Any ideas on why this is or how to get the blade to balance?