What are these black marks around my cat's mouth?

I adopted a 6yo domestic longhair cat a few months ago, and as a first time pet owner I've started noticing this black section around her mouth. It's not visible when her mouth is closed.

I'm fairly sure it's been there since I got her, but I can't be 100% certain. She's absolutely fine otherwise. Eats and bites normally, no signs of pain, doesn't avoid that side of her mouth at all. However, when I got her the shelter mentioned she had some dental disease that meant one of her fangs was extracted.

I've heard of lentigo but that sounds more like spots than lining the gums.

Does anyone know what this could be? Thank you!

https://imgur.com/a/e81QXkr