Normally Batman is fairly strong against psychic and emotional attacks. For example, he fared well versus Scarecrows fear toxin and versus Dr. Destiny's psychic manipulation. Black Mercy got a hold of him for a minute though!

This vid is kind of long, but for those who liked the Scarecrow delusion in Batman: Arkham Asylum (fantastic game!) it's worth watching again:

The game developers did such an awesome job of making you experience the hallucination alongside Batman. The way the screen starts slowly tilting to the side, and how the voices quietly start and then slowly get louder. Finally, you just get lost and confused in the morgue from hitting dead-ends, then turning around and finding the way back was not how you remembered.

Then there was the fact that Bats is hallucinating in a morgue, with all that emotional baggage starting to come out...