A vehicular exterior rearview mirror system includes an exterior rearview mirror assembly that includes a mirror reflective element fixedly attached to a mirror back plate attached to a mirror actuator. The mirror back plate includes a display indicator, fixedly secured thereto, having a light source that is activatable to emit light. The display indicator is associated with a blind spot detection system of the vehicle and the light source is activated responsive to sensing by the blind spot detection system of an object in a blind spot at a side of the vehicle at which the exterior mirror assembly is mounted. During daytime operation of the vehicle and when the light source is activated and with the exterior mirror assembly mounted at a side of the vehicle, the display indicator, as viewed by the driver of the vehicle, is operable to have a luminance of at least about 5,000 nits.