The operator reported that the pilot landed and shutdown the helicopter on an offshore platform. He secured the main rotor system by attaching a tiedown to the forward left main rotor blade and he then attached this tiedown to the left front skid cross tube. Several hours later, the pilot entered the right front seat to begin the engine start sequence. The operator reported that the pilot proceeded to start the engine and noted a few seconds after initiating the engine start that the helicopter made an "unusual noise" and began to shake. The pilot then shut down the helicopter without further incident. A postflight inspection revealed substantial damage to the blue main rotor blade. The operator reported there were no pre-impact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. The Bell Helicopter 407 Rotorcraft Flight Manual lists the preflight check areas in Section 2 Normal Procedures. Regarding the exterior check, this manual states in part; 2-3-B-1. Fuselage – Cabin Right Side 1. All main rotor blades – tiedowns removed, condition. There is also a warning in this section that states, "Failure to remove rotor tiedowns before engine starting may result in severe damage and possible injury."