I'va always assumed that both in the books and in the tv show (but especially in the latter) Cersei blames Tyrion for Joffrey's poisoning because it was the most convenient thing to do: basically she had the chance to get rid of him and she took it. But:
- do you really think that she believes Tyrion is the actual culprit (take a look at her facial expressions during the trial for instance)?
- I don't think so, then I'm wondering why she is doing nothing to discover who really did it. I mean, doesn't she feel endangered?
- same for Jamie... he knows Tyrion is innocent...
- and what about Tyrion himself, or Varys? Do you think Lady Olenna will eventually tell them the truth, now that they are allies? How would they react?