@jtt - quite so. By my quick math it's 1/10th - 1/100th of the weight of a fly's leg. Probably not worth worrying about too much.
I confess to not being too well up on general relativity, but iirc weight (i.e. the force produced by gravity) will increase directly with relativistic mass, so it will affect the weight in the same way it affects mass.
I can't for the life of me imagine any OTHER reason why rotor mass/weight would depend on RRPM. The blade will stretch a little, which will affect its moment of inertia (not by enough to get excited about), but not its weight.