I've always seen a few black specks on the comb, but it seems to be getting worse. could it be from giving them a lot of watermelon rinds and the comb getting wet and sticky and then getting dirt stuck on it? or do you think there's something else going on?
Have you tried cleaning the comb with soap and water or Vaseline to see if it comes off?
Not sure if the watermelon would be the cause, does she have any other symptoms?
Could be something fungal, you could offer her some yogurt, maybe add that extra immunity booster to help her fight off whatever it is, probiotics, Echinacea, apple cider vinegar in the water, extra vits and minerals like poultry spice, or bird tonic drops.
Then of course there is always the option of taking her to the vet if you see the black spot is getting worse.
I get these black spots on the combs a lot. My birds are never affected by it, and I have had it checked out, and there is no reason to be concerned. If it is cold where you are there is a possibility of frostbite, but you just need to put some vaseline on them and they will be just fine. But yes, I have had those black spots before and there is nothing to be worried about Still not exactly sure what it is, but it doesn't affect the birds so I am okay with it.
Is it dirt on the comb, or is the actual comb discolored? It could be dirt collecting due to sticky watermelon or water. I know that my chickens occasionally get dirt stuck to their combs. I'm not sure what else it could be.
Fowl Pox can cause discoloration, but it usually comes in the form of scabs, which I'm not seeing.