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Hello, this is Dr. Jess. I am sorry to hear of your concern with your kitten! Is there any way you can take a photo of her eye? Is she having any discharge from the eye or squinting/rubbing at all?
The most likely thing that is going on is that your kitten has what is called iris melanosis or hyperpigmentation. It would be very unusual for a kitten to have melanoma, which is the cancer you are worrying about. Most of these cases are just a pigmentary change and nothing to worry about, but having a vet examine the area to be sure is a good idea. But don't stress yourself out about it, as it is likely nothing to be concerned about! Here is a small article describing the condition: http://vetbook.org/wiki/cat/index.php/Iris_melanosis