The spell reads in part (emphasis mine):
The mist is too thin to have any effect on vision, but the necromantic energy infused within it hampers the living.
All living creatures within the mist become fatigued and take 1d6 points of cold damage per round.
I assume that since the damage is described as being cold damage it does not count as negative energy and thus does not heal undead but with the spelldescription flavor text figured I would check.