So the black is on the gum, not the tooth? And you're sure it's something new versus just normal pigmentation that's always been there? (Sweep has always had some black pigmentation on her gums, but her teeth are great; the vet's never commented on anything unusual gumwise.)
Re: chews, my vet swears by beef tracheas and lamb ears; she owns 7 greys and tries to avoid doing dentals on them. I buy mine locally, but I think bestbullysticks.com carries them both. Anything like that (i.e., mostly cartilage) should be effective without carrying a risk of broken or cracked teeth. Himalayan dog chews are great for heavy chewers and have a limited ingredient list, but they are quite hard.