... a reacting/blocking vampire cannot play Cats' Guidance (to untap) after the acting vampire has played Mirror Walk (at superior Thaumaturgy)?
The reason for this is, that the action ends immediately when the action is blocked (successfully), so there's no opportunity (for both players) to play either action modifier cards (e.g. Freak Drive) nor reactions cards (e.g. Cats' Guidance).
As a side note: As reacting player you cannot play Obedience either, because Obedience requires that your vampire is "about to enter combat", but the combat is canceled via Mirror Walk's card text.
Reference: VtES Usenet Newsgroup.