If the blue Taoist kills a ghost that blocks his ability to seek help and fight in the same turn, can he then seek help from a villager?
Yes, you can do this.

Under Taoist Powers on page 8 of the rulebook:


  It may happen that a Taoist loses his power because of a ghost
  ability. He can recover it as soon as the responsible ghost is removed
  from his board.


And the blue Taoist's Heavenly Gust Ability on the next page reads:


  The blue Taoist can act in a heartbeat.
  He may request villager’s help AND attempt an exorcism in the same turn, in any order.


So you can definitely exorcise a ghost, unlocking your power, and then get a Villager's help in the same turn.