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 driver-side of the equipped vehicle and that is viewable by a driver of the equipped vehicle. A blind spot indicator is disposed at an aperture established at the inboard wall of the skull-cap mirror shell and provides a viewing area of at least two square centimeters. An illumination source of the blind spot indicator is activated responsive to detection by a blind spot detection system of the equipped vehicle of presence of a rearward approaching or overtaking equipped vehicle. Illumination of the 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 a vehicle forward of the equipped vehicle.