I had Tart of Darkness a couple months ago. It was a good beer but I would hardly call it a sour stout. Its not very stouty at all. It's good though. It's a sour stout just like a black IPA is an IPA. It's dark for the sake of being dark and I didn't feel it had a whole lot of dark flavor to back up the color. I do suggest you try it though it's very good and even better if you just think of it like an ever so slightly roasty flanders sour.
I forgot to add this. If you want a sour beer that is unmistakably a stout, look for Jolly Pumpkin Maduragda Obscura, my spelling is prob wrong but its close enough for goole I bet.