This idiom means something happens very quickly, almost instantaneously. It can be used to describe both positive and negative events.
Examples:
* "The thief stole the purse in the blink of an eye."
* "The time flew by, and the party was over in the blink of an eye."
* "She finished the race in the blink of an eye, winning by a huge margin."
Let me know if you'd like more examples or information on other idioms.