Lots of options here.
Bonus to Move Silently and Hide (maybe add the levels in the PrC to these skills).
Some supernatural ability to counterspell spells with the light descriptor (usable some number of times per day).
Maybe members of this PrC have a better Spot than others so that instead of a -1 every 10 feet, it's a -1 every 20 feet in darkness or shadows. If they're supposed to be all dark and stealthy, maybe part of that is because they see you long before you see them.
Ability to overcome some degree of concealment in dark areas. In other words, if someone else has 50% concealment against them due to darkness, it's only 25% because they've trained to fight in the dark.
Dave