I've used a spot-lamp as the only light source (the fire doesn't spread any light). Also, the problem shouldn't occur in the textures because i already checked it in different angles. If i look at it at the front of this "stair" the black/dark border disappeared. What is exactly the problem and how can i fix this? I've exported this world using jMc2Obj.
The following pictures may be of importance:
Material of the stairs:
Textures of the stairs: