renderer.color.a +=

I'm having a problem with this:

     if(anotherScript.isHurt == true)
         {
 
             Color color = renderer.material.color;
             color.a += 0.01f;
 
         
         }else
 
         {
 
             Color color = renderer.material.color;
             color.a -= 0.01f;
         
         }

Instead of slowly fading the screen black and out it doesnt do anything. I've attached a plane on the screen:

 var bloodObject = GameObject.Find("bloodObject");
 
     if(bloodObject)
         {
 
             bloodRenderer = bloodObject.renderer;
 
             bloodRenderer.material.color = new Color(0.0f, 0.0f, 0.0f, 0.0f);
         }
         else
         {
             Debug.LogWarning("Not found");
         }