A gas turbine engine comprises a fan rotor having fan blades received within an outer nacelle. The fan blades are provided with at least a first type having a first natural frequency, and a second type having a second natural frequency. The fan rotor has a first mount structure intended for the first type and a distinct second mount structure intended for the second type. The first type of fan blade fits into the first mount structure intended for the first type, but there is a first obstruction preventing the first type of fan blade from being placed into the second mount structure intended for the second type. The second type of fan blade fits into the second mount structure intended for the second type, but there is a second obstruction preventing the second type of fan blade from being placed into the first mount structure intended for the first type.