It does, but it also means 'look at the tile below for passage settings'. And if there's no image on the tile, how do you know if it's over or not?

It should DEFINITELY be set to star, so if it's not, go ahead and change that now, otherwise you'll have another issue once this one is resolved.
If you have it set to X, then you shouldn't be able to walk anywhere. If you can, that means you've run an event that's set THROUGH ON for the player, and haven't set it off afterwards.
Or that you've used a different blank tile from the B-E tabs that has a O for passage, and have drawn on all the places where you can walk. If that's possible, grab the top left B tile (set to a star), and draw over EVERYTHING on the map that doesn't already have a B-E tile image.