I took a look at the website (http://www.tgrhelicorp.com/snark.htm) and it doesn not look like they are using a spcial blade. My guess is it they are doing a stagered rotor approach. In which the blades are staggered out of the plane of rotation. So that vortex shed from the previous blade will pass either above or below the following blade. This usually reduces what is called BVI (Blade vortex interaction) What is intresting is that they do not use more then 2 blades, which usually makes for a loud thump thump. the general rule is that the more blades the less noise, since there is less loading per blade.
Hope this helps?
Mike