I seriously doubt cold blue will hide the scratch. Cold blue to my knowledge isn't a filler. If you have ever seen a car paint that has been keyed then you know painting over the scratch doesn't hide the scratch. Also when preparing a gun surface for rebluing the metal surface needs to be as smooth as possible because any imperfections in the metal surface will show up in the reblue.
In my opinion you will be better off leaving the scratch alone or sand out the scratch then refinish. Even then you will probably see the difference in the finish where you tried to repair it.
You can try a cold blue touch up pen, but you still will see a problem with the finish. I say leave it alone and just shoot it and carry it as is because the gun if carried will more than likely get more finish wear anyway.