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Would a two or three-bladed rotor count as a single-blade for the 10% bonus if only one of the blades is covered and the others are left uncovered (without mylar, etc)?

Yes, the other components referred to as "blades" above are not blades as they do not provide lift, they are counterweights. Only the covered surface counts as a blade and presuming all other requirements are met, will count as a single bladed rotor.