A counterweight system for mounting a rotor blade on a balanced rotatable hub of a wind turbine is disclosed. The rotatable hub can have at least one blade root region configured to receive a blade root of the rotor blade, and also have a pitch system configured to rotate the rotor blade around a pitch axis. The counterweight system can have at least one support structure having a proximal end spaced apart from a distal end with the proximal end mountable to at least one blade root region of the rotatable hub. The at least one counterweight mass can be at least partially filled with fluid and coupled to the distal end of the at least one support structure. The at least one support structure can be arranged substantially parallel to the pitch axis such that the pitch system rotates the counterweight mass about the pitch axis.