Balancing long, high speed blades is essential. Even little "dings" in the wrong place will unbalance them. Being long, they need to be "moment balanced", ie you need to take into account their length, not just their weight.
Easiest way is to join them together with a threaded bar through their mounting hole, fixing the blades with a nut each side of them. Make sure the blades are "in line" and that the bar is a tight fit in the mounting hole. Support the shaft (threaded bar, bolt etc) on two (must be parallel) "knife edges", two pieces of sharp metal such as long, straight knife blades or thin metal rullers. Tap the blades to make them rotate. They will stop with the heaviest blade downwards. Repeat this several times to establish that the result is correct. Add a small amount of weight, tape is OK but may come off so you may wish to remove a small amount of the HEAVY blade tip. The nearer the tip you remove material, the less material you will have to remove to affect balance. Keep repeating these operations until the blades, when rotated stop in random positions.