Hi Mako, unfortunately you can't block a specific direction.
That command checks if the pixel 1 step away from the character in the direction specified can be walked on.
So for example, I could have a event run the script
CODE
character = $game_player
x = character.x
y = character.y
d = character.direction
can_walk_forward =
character.pixel_passable?(x, y, d)
can_walk_backward =
character.pixel_passable?(x, y, 10-d)
can_walk_left =
character.pixel_passable?(x, y, (3 * d) % 10)
can_walk_west =
character.pixel_passable?(x, y, 4)
Where the direction is like on the numpad, 2 is South, 4 is West, 6 is East, and 8 is North.
I take it you're looking for something like XP where you could block walking in a specific direction on a specific tile?
Edit:
The working demo
link