No it does damage multiple times jsut liek wall of fire. If you are dumb enough, nsane enough, or a big enough tank to shrug off the damage and decide to stay in the damaging area whether it be fre damage from the wall fo fire or slashing damage from blade barrier it can happen more than once.
And it does an obscenely large amount more damage than wall of fire. Wall of fire seems to take into account its levels, its duration, its additional purposes. And it does what if you are sitting in it 2d6(+1 a level) so at 20th level if it doesn't have a cap ooh, ah 27 points of fire damage. 1d6 per level, 20d6 cap for a cleric spell that is 6th level just shouldn't happen. And no it won't on any obvious level break the game, damage caps by themselves don't do that. The only thing damage caps do is balance spells for efficiency and metamagic feats. For exmaple if fireball had no damage cap, at 20th level its doing 20d6 wouldn't break the game because you can dish out 20d6 spells anyways. It does break the spell though because it cost you a lot less in resourses, than a 20d6 spell should and if you decided to empower it or maximize it would out damage the other higher level spells you could have access to and likely by a good bit. Same thing applies for balde barrier though on a less dynamic scale. You get more bang for you buck than you should out of a 6th level spell(20d6), and if you decided to empower it(effectively 30d6 for 8th level) or maximize it 120 points of damage at (9th elvel spells) it then outdamages other spells of its new level.
