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There are a number of things that could be causing the discoloration on your cat's gums. It could be caused from plaque (cats need their teeth cleaned just as we do), it could be the early stages of gum diseases (gingivitis) or the early warning signs of oral cancer or FIV. Any of these can cause black looking gums in your cat. The safest ay to determine the problem is to have your vet exam her. It could be that she simply needs her teeth cleaned but if it is something more serious you have caught it early enough that treatment may be possible. I am sending you several sites that discuss feline oral care, diseases and possible treatment options.
I know this seems like a lot of reading but the more you learn the more you can protect your furbaby from preventable problems.
I hope this helps. Please let me know if you need additional information.