I was confused about the discrepancy as well. I think we're on track with this (course, all could change and there could be a bug that proves me wrong):
Nightshade (one word) -> non-tradeskill item
Night Shade (two word) -> tradeskill item for alchemy in various potions (for shammies)
Nightshade (one word) is foraged.
Night Shade (two words) is purchsed through certain vendors
Oggok: Garakbar, Plane of Knowledge: Severg ODanos, Thurgadin: Hulda (at the time of this post).
Night Shade is not used in poisonmaking (often assumed so by myself and most), it is, however used in for variations of purity and poison awareness potions (alchemy).
