Alright. I've come up with what I think is a reasonable explanation.
Fact (as in, the lore as it exists):
The schism happened when elves cast out those who were greedy, evil, malicious, etc; they were referred to as to "dark elvenfolk".
Say that this is/was a colloquial term. that referred to the color of their heart and not their skin. It was through their worship and service to Lolth that their skin grew as dark as their hearts. (or some other flowery word crap).