Bloodspine Wall does not block Line of Sight. If you make a ranged attack through a Bloodspine Wall, you resolve the attack normally. The wall does not take damage from that attack. You could also choose to make a ranged attack against the wall itself, in which case it would take damage. Think of Bloodspine Wall as a wall that is short enough that you can shoot over it, but high enough that it is still an impediment to movement.