I'm new to all this too, but I think the CG of each blade is what you're after. That is, if you stuck the two blades on a balancer, you'd be able to guarantee that both weigh the same and have roughly the same weight distribution from base to tip, but for the real stickler's out there who want to eliminate as much instability as possible, the CG of each blade has to be tuned and matched -- individually, and across both axices (base-to-tip and leading-to-trailing edge -- even top-to-bottom if you're crazy 'nuff).

It's one thing if the two blades weighed the same, but finding and tuning the CGs takes balancing the blades along each axis. I've used a taped-down round metal rod on a flat & level plastic surface to do some rough CG matching. I set the blade down on top of, and perpindicular to, the rod and nudged it back and forth with a pen until I found the center along that axis. Rotate 90° and repeat, and again repeat those two steps on the other blade... Then compare it to the first. That's when you get to do the real balancing of the blades.

You can see that if the blades need to weigh the same, and need similar CGs (if not idealized CG's for optimal performance), and you've used glue or tape to do repairs, that it'll be difficult to balance all these parameters out if one (but not the other) of those parameters (weight and CG) is askew... if that makes sense