Barring frostbite, her comb could have suffered an injury. One of the nifty things about chickens is they can injure their comb and feet and the tissue will be destroyed and dry up and fall away with little to no effect on their body.
I doubt very much the hen would "get stressed and die" if caught and examined, which is definitely what I would do. You need to inspect this anomaly up close and figure out what it is. You might be surprised it's something you weren't expecting, which would really rile up my ever present curiosity.
You have the advantage of being able to physically examine her and to use your senses to help describe it. We can only look at a two dimensional photo. By the way, please try to get a better photo that is well focused so we can see more detail. I need to see what the blackened spots might be, although it's usually a sign of necrotic tissue, and that will be what will eventually fall away on its own.
Take a scraping and tell us about it. Dab a bit of oil on the white area and see if that changes the color. What does the comb feel like, soft, hard, crusty, bumpy? Does the hen try to scratch it?
