I don't think this is possible.
If you can't make transparent textures for the blades (like me), or don't have the skill to do so (again, like me), use the textures that come default with the game.
Here is a list of them, they can be found in Data.pbo:
vrtule_uh_m_bl.paa (4 bladed blur)
vrtule_uh_v_bl.paa (another 4 bladed blur)
vrtule_velka_bl_.paa (2 bladed blur)
-Student Pilot