Blood Elves (Sin'Dorei) were High Elves (Quel'Dorei) until the attack on their lands and their Sunwell. One group chose to not look back at their losses and seek to adjust (Quel'Dorei) and the other, chose to look towards their losses. And their loss of their Sunwell. Chosing to take the name Sin'Dorei to remember their fallen, they followed Keal'thas in hope to find a new source to take the place of the Sunwell. Sadly this was a bit idea, the fel energies were an addiction to the people, and the energies surging through their bodies is also the reason to the green glow of their eyes. The addiction for more power made the Sin'Dorei hope to find new waits, all from Fel crystals to sucking the mana energies out of the beings around.
Though this is now a bit out of date due to the reclaim of the Sunwell, and cleansing. The green mist should soon disappear from them, unless they keep their addiction up.
((That is just what I know))