Blizzard. Has. Directly. Said. It. Night Elves evolved from Dark Trolls.
Please give me the link so that I might bow to your superior knowledge of Lore.
Because the only two OFFICIAL references I know about, is the book in Booty bay (and elsewhere) that states that *Trolls* believe that Night Elves came from them, via the Dark tribe that went to the well, and the old Troll info webpage from the old site that again states that *Trolls* believe that Night Elves came from them - as a matter of explaining why they lost so much territory to them. They even added at the bottom that this is a matter of conjecture, and no Night Elf could fathom such a thing being true - and that there is no proof.
No where has Blizzard come out and directly stated that Trolls spawned Night Elves.
For the most part it's "fan-made" lore - that Blizzard loves to play with, but has NEVER confirmed in any official manner.
But if you have a quote, I'll accept that.
Blizzard has come out and directly stated that Night Elves were Dark Trolls who, after exposure for many years to the well of eternity, slowly turned into Night Elves.
Same thing happened to the highborne and the sunwell. They got shorter, blonder, etc.
It was said officially and directly (what ever the hell "directly" is supposed to mean) in the WoW magazine.
No one is going to be able to find you a link because A) the magazine died, like, two months ago, and B) as far as I know there aren't digital copies of the magazine that you can view for free.
Seriously though, go on any reputable WoW site (like MMOchamp) and use the search feature, it was literally everywhere.
EDIT: On top of that, it was recently, it was definitely sometime earlier this year that it was mentioned.
EDIT#2: "At the beginning of the world, now known as Azeroth, the god-like titans created a magical well — dubbed the Well of Eternity. According to Freya, Cenarius, and the Tribunal of Ages, it was a tribe of dark trolls that eventually evolved into the Kaldorei night elves."
Volume 2, Issue 1 of the WoW Magazine.
Thats as good as its going to get.