I have dark brown hair and brows, and I use Dark Edge to fill mine in... don't know what I'm going to use when it's gone though! Maybe try Espresso, or if that's too dark, Cork.
My favorite MA filled in my brows with Mystery (would never have chose it ) and since I'm hooked. SO yeah one more vote for mystery, I have black brown hair right now, worked with my black and brown hair too.
I am NW 15 with brown hair and a tad lighter brows, I use Coquette and it looks really natural - if Coquette was too light (or maybe I wanted a bolder look), I would try Mystery.