The pilot had just picked up an external 'sling' load of 2700 lbs of back-fill in a cement bucket. At 900 ft AGL, the aircraft started a vertical vibration. As collective was bottomed, a large piece of one main rotor blade separated. The pilot said he jettisoned the load and the 100 ft line and landed the aircraft in a mountainside clearing. During the post-accident examination, a portion of the main rotor blade about 32 inches long was found missing. This failure started with a crack on the blade. While on the ground, the drooping of the blade would tend to close and mask the crack on the bottom of the blade where it would more readily have been seen. A general aviation airworthiness alert stated that fractures on helicopter blades occur at locations where blade damage existed. Damage ranging from scratches, dents, nicks, or corrosion pits became a potential point for stress to concentrate and propagate into cracks leading to blade failure.