Elves that made Quel'thalas didn't evolve from trolls, those evles that we call Blood Elves are part of Night elves Highborn and they became light skinned due to further usage/practice of Quel'Thalas magic, since they created their own well in there to fuel their addiction to magic. After Sundering and banishment of Hihgborn from Night Elves society, those Highborn that didn't want to stop practicing magic took ships and left what is know today as Kalimdor and found new land to settle in EK.