Many of the lift and cruise concepts for eVTOLs want to stop the propellers in forward flight. A single blade propeller may be usefull, because it could implement a flapping hinge, while still avoiding divergence of the forward facing non-rotating rotor blade. Implementing a flapping hinge would allow to reduce the mast moments, which can be quite high on a rigid propeller.
@Ascend Charlie: The aerodynamic moment can not be transferred with a Flapping hinge that is located in the rotor center. The moment is reacted by a constant flapping angle (conical rotor blade "disk"). The blade goes up and the counterweight goes down. The centrifugal force acting on these two weights creates a moment that counteracts the moment from the non-central aerodynamic forces.