The blue Taoist has the ability to request help from a villager and fight a ghost in the same turn. If this ability is blocked by a ghost, but then that ghost is killed by him, can he follow up in the same turn and request help immediately since his power is now unblocked?
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.