Gavinmad
Mr. Poopybutthole
After considering the shitty writing for this season, I'm pretty confident that it's fake blood, despite what I mentioned earlier about that being completely impossible without the person stabbing you being in on the trick. Would explain why she was so publicly walking around as Arya Stark buying passage back to Westeros and how she's up for such vigorous activity in the next episode. She could be staggering around the marketplace trying to lure the waif back out or something, even though it would make much more sense to just pretend that she died.