Surprising,i’m having a hard time finding daggers with lightning effects on em.
Can’t you just transmog mongoose on em
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There’s a weapon from one of the Vol’dun dungeons but it’s a Sword sadly. 
I don’t remember if there’s a dagger version of this.
https://www.wowhead.com/item=158373/resonating-crystal-scimitar?bonus=4777:1473
https://www.wowhead.com/item=158714/swarms-edge?bonus=4777:1473
Ulduar drops swords. No clue if daggers.
axes too from there have lightning.
Colossus works better if you want the traditional electric blue look.
If you can get it to drop!!