Black/dark blue line in my gums. Is this something to worry about?

[bayareazfinnest]

This morning I noticed a thin black/dark blue line running down my gums in between two teeth. It slowly fades as the line goes down the gum. My mouth does not hurt or anything, and I take care of my teeth fairly well (except don't really floss as much as I probably should). Is this something to worry about? If so what could it be?

Thanks!