Black Spot on Tooth Near the Gumline?

I have had a small, circular black spot on my lower left canine near the gumline for the past few months. It's not causing me discomfort and I can easily brush, floss etc. the area, but it's very unsightly and a bother to look at. I have tried to rinse with a xylitol based mouthwash and brush with a whitening toothpaste, but haven't seen any improvement. Is this a cavity?