Quick Query - Complete Arcane Feat and Spell

The feat Black Lore of Moil causes necromancy spells to deal extra negative energy damage, but does anyone know how this works with duration spells? Does it keep doing the damage each round or is it just a one-shot effect?

Specifically, I've got a high-level lich planning on using Black Lore with the spell Sword of Darkness, which creates a floating sword that attacks people and inflicts negative levels. I'm assuming the sword does the extra damage each time it attacks, but this strikes me as particularly nasty...

And I need an answer by this time tomorrow...

EDIT: To be clear both the feat and the spell are from Complete Arcane

In the third paragraph, the feat states that the maximum energy damage dealt is based on the spell's level. That implies to me that it only deals damage once--on the first successful attack made by the sword of darkness spell.

I agree with CM. The extra damage is per spell, kinda like how sneak attack damage is added only to the first round of an Acid Arrow.