I used it on a few scratches. I had a deep one that was down to the primer but not the metal (you could feel it with your finger). It has done a great job but the scratch can still be seen, but I would say most people would not even notice it, I would say no one would be able to see it unless they were looking for it.
For smaller scratches its a miracle. I used black t-cut on my black car so I dont know if you will get the same result in a blue car.