Fluff is irrelevant to rules, because there are a number of rulebooks that suggest strongly that players should refluff. Could the Lightning Daggers be physical daggers? Sure. Could they be bits of a Lightning Soul Genasi launched at enemies? Sure. Could they be electrical shocks to an enemy's heart? Sure. Could you frame all of your lightning attacks and damage as fire (as long as you still use the items/feats/etc. for lightning attacks and damage)? Sure. That's actually a big difference between 4e and previous editions, and it's one of the reasons a number of people disliked it- they wanted to be told how to interpret their actions, while WotC basically said "as long as you follow the rules, describe it how you want."