Blink object on collision

You're close, but missing one crucial detail.

Unity's docs say that StartCoroutine returns "immediately". The function making the call doesn't wait. It's only the coroutine itself that waits.

You could probably re-organize it something like this:

 void BlinkPlayer(int numBlinks) {
     StartCoroutine(DoBlinks(numBlinks, 0.2f));
 }
 
 IEnumerator DoBlinks(int numBlinks, float seconds) {
     for (int i=0; i<numBlinks*2; i++) {
     
         //toggle renderer
         renderer.enabled = !renderer.enabled;
         
         //wait for a bit
         yield return new WaitForSeconds(seconds);
     }
     
     //make sure renderer is enabled when we exit
     renderer.enabled = true;
 }

Edit: It occurred to me that it might be easier to control the coroutine by duration (in seconds), as opposed to a fixed number of blinks. Something like this...

 public class Blink : MonoBehaviour {
     void Start () {
         StartCoroutine(DoBlinks(3f, 0.2f));
     }
     
     IEnumerator DoBlinks(float duration, float blinkTime) {
         while (duration > 0f) {
                 duration -= Time.deltaTime;
      
            //toggle renderer
            renderer.enabled = !renderer.enabled;
      
            //wait for a bit
            yield return new WaitForSeconds(blinkTime);
         }
  
         //make sure renderer is enabled when we exit
         renderer.enabled = true;
     }
 }