Hey i have some black thing on the back of my bottom front teeth. i tried to scrape some of it off and now i get a gritty feeling when i run my tongue over it. any ideas?

The back of one's: bottom teeth is a common place for plaque and tartar to accumulate. Decay may be associated with darkening. Perhaps discolored plaque or decay is what you saw and by scraping it, possibly with something too abrasive, you have disturbed the enamel. Appears to be time to see your dentist for a diagnosis and up to date prevention. Good wishes:)

Tarter: What Dentists call Calculus. Hard deposits on your teeth from calcium in your saliva precipitating out into bacterial that hasn't been removed. Tarter will cause gum recession and can be quite damaging. Dark color may be from gums bleeding into the tarter. Please see Dentist/Hygienist for professional cleaning and oral hygiene review.

Sure: Sounds like stained calculus. You'll need the services of a local dentist to remove it all. Make the call...