I doesn´t work that way. When it´s dark and you flash your light a person would have a reflex to close the eyes for a short moment and turn the head away. That will limit the blinding effect and your time to react. The time span is closer to one second than to a few seconds. Getting the flashlight ready for use may cost you more time than you get from using it.
Marc 'Animal' McYoung has a pretty good article about how to avoid getting into a selfdefense situation. The site is about knife fighting but you can skip that part if you´re not interested in that.