Just another point of interest in this discussion about numbers of blades- the Phantom biplane racer at Reno gained over 30mph going to a 4 blade Paul Lipps design compared to the original 2 blade. Absolutely huge.
This shows that with proper design, more blades does not always have to be slower.
I submit that some of the 3 blade props available for RVs could be improved on.
Assuming the prop is properly designed and optimized for the aircraft.....
I believe the theory is that less blades are optimal because each blade has more time to clear the turbulent air from the previous blade sweep. This works up to the point where the blades cannot support the power of the engine. They either have to become too thick or too long or run too fast. Then you have to shift up to more blades to effectively absorb the power.
Because the RV-10 is a relatively fast aircraft with a powerful engine, I suspect that the 2 vs 3 blades argument is finely balanced.