How does the Eurocopter Blue Edge and Blue Pulse rotor blade design work?
EC155 helicopter. Compared with a conventional blade, the Blue Edge has a tip with both forward and rearward sweep. 
This gives the blade more time to interact with the vortex 'shed' by the rotor, reducing the characteristic impulsive (slap-slap) blade-vortex interaction noise generated by a helicopter.
The active trailing edge flap harmonises the frequency thus making it quieter.