In Old Blackburrow (instanced zones): Had a chance to spawn in group mission zones.
In Oceangreen Hills: The Blackburrow Nightstalker is known to spawn in the northeastern part of the zone (click for map image). Its spawn mechanism isn't known for sure (either on its own timer as a night time spawn, or one or more of the "a Blackburrow lurker" NPCs are PHs).
One known spawn area is /location 4650, 25.
It hits for a max ~2,350.