Well, considering that the whole exorcism thing was fake, why is it that Tara saw the blood on the uh.. stake? Or whatever it was. Were the meds messing with her mind so much that she could see "physical" evidence?
peyote is an extremely powerful hallucinogen... she probably just imagined the blood on the stake thing.
I agree with "killface" - I think she saw the blood due to the peyote and because her mind "wanted" to see the blood after she stabbed the girl she hallucinated as having been there. Stab a person, see blood on the knife....make sense?