NCF – Extended list of Neutral creatures.

Was ABILITY_BLADE_BARRIER originally created by the developers and not modified by players? None of the standard creatures have it, and it's not listed anywhere. Also, it seems too powerful compared to other abilities.

Maybe you should play the game sometimes instead of just creating mods and browsing creature abilities.
ABILITY_BLADE_BARRIER is actually used by the "Blade Barrier," which is the Wall of Blades. Which, of course, is not a creature. It's a spell from the Summoning school. It's designed for the AI to attack and waste its resources on it (the wall).