what's this black stuff on my teeth

So I brush my teeth twice a day. I drink 1-2 coffees a day. And red wine maybe once a month. I keep getting these black stains that are SO hard to get rid of. Brushing won't do anything. They go away after a dental cleaning but soon reappear. I only get it on the bottom teeth, and I'm wondering if it could be this bar behind my teeth. It's been there for like 8 years already. I'm so annoyed!!!!!