How do I make a block like minecraft where it gets...

In Minecraft, the hit counter is not reset when the object gets offscreen, but rather when the cursor (center of viewport) is no more over the block. The obvious advantage of doing so is to need only one counter instead of one for every single block.

Just cast a ray from the center of the screen in the camera direction, and check when the target changes:

 var currentTarget:GameObject = null;

 function Update() {
     var ray = camera.ViewportPointToRay(new Vector3(0.5F, 0.5F, 0));
     var hit;
     var newTarget:GameObject = null;

     if (Physics.Raycast(ray, hit)) {
         newTarget = hit.collider.gameObject;
     }

     if (newTarget != currentTarget)
         ResetHitCount();

     currentTarget = newTarget;
 }