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.