An exterior rearview mirror system for a vehicle includes an exterior mirror assembly having a skull-cap mirror shell that includes an inboard wall that faces generally toward an exterior side of the vehicle and that is viewable by a driver of the vehicle. A multi-stage blind spot indicator is disposed at an aperture established at the inboard wall of the skull-cap mirror shell. An illumination source of the blind spot indicator is activated to illuminate the multi-stage blind spot indicator responsive to an electronic blind spot monitoring system of the vehicle detecting another vehicle that is approaching or overtaking the equipped vehicle. Illumination of the multi-stage blind spot indicator by activation of the illumination source is readily viewable by the driver of the equipped vehicle and is generally not viewable by a driver of another vehicle traveling in front of or approaching from in front of the equipped vehicle.