Did the gearbox fail causing high gyroscopic loads on the prop disc causing one blade to fail
I believe that any gyro loads have to react at the center of rotation with the gearbox firmly attached as ground. About the only way you can transfer these loads into the blade would be if a large mass like the aircraft itself should alter the axis at a high rate and react the load through the gearbox to the blades.
In that case all blades would be severely distressed in a similar fashion. I'm betting the aircraft tail would never survive such pitch or yaw loads.
By now the investigation should have pin pointed the exact location of the fracture initiation and the time it took to progress to fracture as well as the frequency of the cyclic loading, arrest marks etc.