The ability requires you to see an unoccupied space, but not through that space. You can teleport to any unoccupied space on the edge of the darkness spell effect because you can see the space of darkness (just not the through/past it).
For comparison, you don't have to see the entirety of a creature to target it with a spell that requires sight, just a single face of it, or even a single square inch of its surface. Why would targeting a space be different?