Marmite Surprise

When a girl has black poop in between her vaginal lips (labia) but the outside is clean. It's a "Marmite Surprise" when you spread her lips.

John: How did your date with Sarah go? Did you score a home run?
Dave: Yeah dude, it was great. Even got a Marmite Surprise! I didn't even know she was into that sort of thing...
John: Aw man! You always have the best dates!