The fake blood takes a huge leap of deductive reasoning on Arya's part
1) Arya assumes the Faceless Men will kill her for failing, because JH pretty much laid that out
2) Arya would probably assume the Waif would do it. The Waif isn't exactly subtle about her hate
3) Arya knows the Waif hates her, so she will choose a more personal method of assassination vs. a less personal one (ie, poison or long ranged)
4) If the Waif will choose a personal method of assassination, it will be close quarters and let Arya know who killed her
5) It would probably be a "slow" death, so she would first aim to incapacitate Arya/inflict a mortal wound that isn't immediate. Given Arya's experience, a stomach wound (the dying farmer when she was with the Hound)
6) Arya deducts this will be a stomach stab, so she rigs some complex system that will bleed on puncture and feel like flesh so that the Waif thinks she really stabbed Arya
7 ???
8) Profit
I can buy the first few leaps of logic, but then you start to get into Sherlock Holmes/PreCog level of prediction.