Something black on my tooth, what is it?

on my very back molar, the ones that come in as you grow older, has something black on it. i noticed it for the first time today. and today is also the first time i've been having an ache. i've tried brushing it away and flossing. i dont know how this happend i always brush my teeth and floss. and i've been going to the dentist regularly. if you could give me some input that would be great, thanks.