The Sunwell Redemption
The final tie between Kael'thas and Quel'thalas was broken when Kael'thas' minions returned to take back by force the naaru, M'uru, which he had once given to his kindred so freely. Lady Liadrin and her Blood Knights would have been left without any of their powers had the naaru A'dal not reached out a hand to save her and her people. Lady Liadrin was deeply regretful of what she and her Blood Knights had done to M'uru, but A'dal forgave them, saying that M'uru had known all along what his role would be in this unfolding drama. The naaru extended his own Light energies to Liadrin and her Blood Knights, and encouraged them to assist him to stop the terrible threat that Kael'thas now represented to all the people of Azeroth and Outland.
The former "Lord of the Blood Elves," now quite insane, had brought the remaining strength of his forces back to Azeroth and taken over the Sunwell Island, just across the channel from Silvermoon City, and planned to use the hidden energies of the Sunwell's magic to try and summon Kil'jaeden into the world. The blood elves and draenei of Shattrath united to overcome this threat, and as their forces ventured deeper and deeper into the Sunwell fortifications Kael'thas had set up, they found that M'uru himself was enslaved as a guard the site where Kil'jaeden would be summoned. The heroes were forced to destroy his weakened body and stop the entropic energies which now began to vacuum up all life around it as the last of his Light energies seemed to drain away.
At last, of course, the heroes faced Kil'jaeden himself at the site of the Sunwell (perhaps your own character was among them), and, with the help of some dragons, they drove him back into the Sunwell Portal, away from Azeroth. The draenei prophet Velen arrived, along with Lady Liadrin, and spoke to the heroes, as he placed the last remaining fragment of M'uru's body into the Sunwell. The result is one of the best scenes in Warcraft lore, which you too can look on, as the last spark of M'uru's life reignites the Sunwell with the energy of the Holy Light, restoring once and for all, that magical life energy the blood elves need, as well as something far greater, something with the power to rebirth the entire civilization of the blood elves: Faith.
Answers and Questions
The implications for blood elf characters are enormous. First of all, apparently the blood elves are no longer in magical withdrawal. They certainly no longer need to feed on demonic energy in order to sustain themselves (although their eyes remain green, perhaps as a mark of the taint that used to flow through them). The magic of the Sunwell has historically been broadcast to all the high elves (and now blood elves too) in the nation, and seems to do so even more now than ever before. The Sunwell has never burned so brightly, with so much pure and holy energy as it now has.
Likewise, the Blood Knight paladins no longer need to "steal" any Light energy away. It was a free gift of the naaru for a time, and now the Light is an essential part of the Sunwell itself. The Light is right there for any blood elf who wishes to access it. In this light, you could even interpret their modified Arcane Torrent ability (now minus the "mana tap" part) as an explosion of the Light-based energy flowing through them in addition to regular old magic. It makes perfect sense that most Blood Knights could have genuine faith in the Light now, and may indeed deeply regret their behavior in the past, trying to steal that which the naaru actually gave away freely.
A fair number of blood elves are sure to be reluctant to give up demonic magic, however. Kael'thas couldn't have been the only one to be driven mad by it. Doubtless, many of the most wicked and power-hungry blood elves could have joined the Blood Knights in a quest for more power; but now that their leader, Lady Liadrin, has embraced the Light for real, they may feel distainful of this new direction for their people. If your character looks down upon all this fluffy do-gooder stuff, he or she may try to steer other blood elves back towards the way things used to be under Kael'thas... perhaps not so far as to turn the world over to the demons or anything, but perhaps just enough to make as much use of demonic power for personal gains as possible.
Still, all these events leave us with far more questions than answers. Since the events of the Third War, there has been one turning point after another, as elves have gone from noble "high elves," to members of a crushed civilization, without hope or faith of any sort, through stages of greed and lust for power, remorse, and now... what? Does your character feel the power of love and grace emanating from the new Sunwell just as Lady Liadrin does? What must such a power feel like after so many years of addiction to mere arcane energies, followed by a dangerous trist with demonic magic? To what degree was your character involved in the injustice inflicted upon M'uru, and to what degree does your character now seek to make good of those deeds now that M'uru's spark of life has rejuvenated the greatest symbol of your people, the greatest fountain of magic in all of Azeroth, and quite possibly the soul of your nation? What does it mean to be a blood elf in this new age? What direction will they choose from here?
These questions are complex, and can be answered in many ways. Indeed, most of it is so new that many players, just like blood elf society itself, are still formulating their own answers, and many disagree on various issues. It fits though -- blood elves are a people in midst of a great whirlwind of change. But as the story of Warcraft moves on and turns to face its inner demons in Northrend, consider what new resources of faith and hope may be growing within your blood elf character. Consider also what challenges and difficulties may await him or her, which may severely test that faith and hope once again.
For further reading about blood elves and many of the implications of playing one, check out Blogatelle's in-depth coverage of the questions surrounding blood elves, and stay tuned for more to come about roleplaying all races and classes in World of Warcraft here in All the World's a Stage.
[Edit: I put in a few more paragraphs under the "Answers and Questions" heading and made a few other changes on this page in order to clarify a number of issues that readers brought up in the comments. I had previously thought many of these conclusions would be self-explanatory, but they really are such a big change from the way blood elves were originally presented to us that they deserve clarification.]