Hi. Is the spot raised, or flat and just looks like darker colored skin? Feeby (15+ yo) has a couple of completely flat, darker colored spots on her ears. They have been there for quite a while now, I just assumed they are age spots like humans can get. But, hers are closer to the tips of her ears where the hair is thinner. If that spot is a bit raised and/or you can feel a difference in the texture of the skin there as opposed to other places, it could be a small wound - or, perhaps even a mole or a wart?
I would send a pic to your vet and ask what they think, especially if your cat is fairly young.
Hopefully, other members will come along soon and have some ideas to share with you!!