ok this spell is  amazing because it just has a ton of potential. our party cleric has this on him, and both times he's used it for absolutely ridiculous things. first time he used it to stop a freaking landslide and the second time he used it to bisect a giant enemy in a one-on-one trial and kill the thing in two rounds. clerics are absolutely terrifying and amazing at the same time

The fact you can make this wall up to a hundred feet long is ridiculous. 

*Evil DM grin*

Question: if this spell was cast on a legendary creature, would the creature be able to avoid damage from this spell altogether by using a legendary action to move? 

The Pale King turns in his grave.

Yes. As long as it moves out and doest enter it again. Since it happens in the 6 second window, picture the blades forming as the monster moves.

Quote from Gaines7922 >>

Question: if this spell was cast on a legendary creature, would the creature be able to avoid damage from this spell altogether by using a legendary action to move? 

 No. Legendary actions are taken at the end of another creature turn. The blade barrier would already be online by then.

Quote from Gaines7922 >>

Question: if this spell was cast on a legendary creature, would the creature be able to avoid damage from this spell altogether by using a legendary action to move?

No, the damage is taken the when it enters the area for the first time on a turn, not its turn.

That's what I thought and ruled.

Quote from WillJewwel >>

Quote from Gaines7922 >>

Question: if this spell was cast on a legendary creature, would the creature be able to avoid damage from this spell altogether by using a legendary action to move?

No, the damage is taken the when it enters the area for the first time on a turn, not its turn.

 According to the tweets of the high muckety mucks and the sage advice compendium a spell's effect appearing on a square does not cause a creature to "enter the area for the first time"; that refers to a creature using their movement to enter the area of effect of the spell. See: https://media.wizards.com/2019/dnd/downloads/SA-Compendium.pdf#page=16 This works the same as with moonbeam, cloud of daggers, and spiritual guardians, which all use the same language to describe the damage triggers.  A creature within the area of effect of a blade barrier, moonbeam, etc can be saved from taking damage if they are moved out of the aoe before they start their turn.  Similarly, since legendary actions take place prior to the creature's turn and since the creature would not be using movement to enter the area of the spell the creature should not trigger the damage if they move out of the aoe with a legendary action.  For further discussion of this as well as copies of crawford tweets stating moonbeam does not do damage on the turn it is cast and that the intent is that casting a spell on an area where a creature is located does not cause that creature to "enter the area for the first time" see: https://forums.giantitp.com/showthread.php?472042-Does-Moonbeam-Do-Damage-When-Cast    Note, however, that if a creature who is not inside the area of a blade barrier (or moonbeam, etc) uses a legendary action to move through the blade barrier (or moonbeam, etc) they do take damage since they are using their movement to enter the area for the first time. See: https://www.sageadvice.eu/2016/06/09/if-a-monster-uses-a-legendary-action-to-move-through-a-moonbeam-does-it-take-damage/

Quote from Wienerbarf >>

The Pale King turns in his grave.

 Nice hollow knight joke

Have a wizard cast wall of stone around an enemy (with an arrow slit so you can see inside), then have a cleric cast blade barrier inside it and watch the enemy die.

Why is this a Cleric only spell? It feels very Wizard/Sorcerer...

So is the slashing damage from it magical? I assume so since it is a spell, but still.

If you cast it between your allies and the Enemy, does only your allies gain the three-quarters cover benefit? Or does the enemy gains it too since it's in the other side of the barrier?

My guess is that, since it doesn't specifically say that enemies don't get cover, everyone within does. It also wouldn't make logical sense for them to be perfectly able to be seen, since it's not that type of magic from what I read and it's basically just a wall/cone type thing. To conclude, probably not, but it's your game, do whatever tf you want, with the DM's discretion and/or fairness in mind.

Quote from RenAndXert >>

Have a wizard cast wall of stone around an enemy (with an arrow slit so you can see inside), then have a cleric cast blade barrier inside it and watch the enemy die.

 That's basically the "slumber party in the breakroom" from Your Pretty Face is Going to Hell.

I think its like Wall of fire, etc. There is a friendly side and an enemy side. As a level 6 spell, I'd say the enemy side (those not "behind" the wall, that being those protected by it) don't get cover. It's magic.

What if a Cleric casts Blade Barrier, and a Wizard casts Scatter to forcibly teleport enemy creatures into the Blade Barrier? Would the creatures then take the damage once for entering, and once again at the start of their turns? Or is there some ruling that being forcibly teleported doesn't count as "entering the area for the first time" either?

IKR

Quote from YourBoiCyan >>

What if a Cleric casts Blade Barrier, and a Wizard casts Scatter to forcibly teleport enemy creatures into the Blade Barrier? Would the creatures then take the damage once for entering, and once again at the start of their turns? Or is there some ruling that being forcibly teleported doesn't count as "entering the area for the first time" either?

They would take the damage as they have entered the area for the first time on a turn. They would then take it again on the start of their turn as well.