Reading this answer, I realized that the balance of a rotor is critical. When one blade is changed, the whole balance of the rotor may be affected. Moreover, I doubt 2 blades are exactly the same. For a car's wheel, we add small weights, but what about helicopter's rotor? Adding pieces to the rotor may affect its aerodynamics.
Most blades are as close as possible when built at the factory. Yes, it is like a car wheel you use weights to balance. For the Blackhawk helicopter there are inserts on the hub near the blade to add weights. But additionally there are a maximum amount of weight that can be added. If you're unable to balance the main rotor head, sometimes you have to switch blades around to other sides and adjust weights. If it still doesn't balance usually it is due to the installation of the hub to the main transmission then gaps have to be measured.
Mass balancing of helicopter blades is pretty much the same as any other rotating component- by addition of masses. Usually, unbalance in the helicopter rotor disc results in lateral vibration- requiring shuffling of masses in rotors for reducing it.
In general, the rotor blades are balanced on ground (statically balanced as a set, usually), so as to reduce the deviations caused during manufacturing process. After installation on the helicopter, the dynamic balancing of the rotor blades is carried out to bring vibrations within a set limit. This is done by addition or removal of masses, usually near the hub area (though this is done near the tip in some helicopters). This should bring the vibration within limits in most cases- if not, this is achieved by replacing the blades individually, or in extreme cases, by changing the whole set.
The masses are usually added near the hub, where the aerodynamic penalty is lesser or adjusted within mass chambers, which are not exposed, thereby reducing the adverse aerodynamic effects.