A new civilization
These rulers knew that the Highborne could not be allowed to continue influencing and harming their community in this way, but also didn't want to simply put their kindred to death. They decided to banish the Highborne and their leader, Dath'Remar Sunstrider from their new homeland in the forests of Kalimdor.
To the night elves, Dath'Remar was a dangerous terrorist rebel, but to the Highborne, he was a great leader, and the founder of their new civilization. He led them across the sea, past the wild Maelstrom, to the northern lands of the Eastern Kingdoms. There, they encountered the humans and had peaceable relations with them, but they had to fight tooth and nail with the Amani forest trolls for every inch of the land they wished to occupy. Finally, they eventually established the kingdom of Quel'Thalas, and created a new fountain of magical energy called the Sunwell, using a vial from the original Well of Eternity that had been destroyed in the great war.
This time, however, they had some warning of the dangers involved in this use of arcane magic, and they attempted to use it responsibly. They placed monolithic Runestones on the borders of their kingdom in order to shield their use of these energies from the extra-dimensional demons. Over time, the energies of the Sunwell became an integral part of their identity, their culture, and even their own biology.
(Although their elven name, "quel'dorei," literally meant "Children of Noble Birth," they came to be known in these Eastern lands as the "high elves.")
These magical energies transformed the high elves into something quite different from their kindred in the far West. Without the World Tree, Nordrassil to bring them immortality and total harmony with nature, their stature shortened, and their complexion turned paler, and their eyes began to shine sky-blue rather than yellow or moon-blue. These changes seemed fitting, however, as they had already started to think of themselves as people of the sun rather than the night, and had completely separated themselves from their old identity.
Over thousands of years, they forgot much of their ancient history among the night elves. They became increasingly intertwined with the humans and dwarves in an Alliance against the trolls, and even against the orcs who make their way through the Dark Portal and up into Lordaeron.
A new catastrophe
Your blood elf character would have started life thinking of himself as a "high elf," a member of this peaceful civilization which did not worship nature, but certainly lived in harmony with it. You would have thought of humans and dwarves as allies in your struggles against the trolls and orcs. From the time of maturity around the age of 60, or even until old age around 350 years old, it would seem to any normal blood elf that things could continue as they always had. But the arrival of the Scourge, just 5 years before the current setting, changed everything -- ending the centuries of relative peace and prosperity that the high elves enjoyed.
Arthas and his legion of undead marched through Quel'Thalas, destroying the protective Runestones, and leaving a strip of lifeless wasteland in their wake. Arthas murdered their Ranger-General, Sylvanas Windrunner, massacred about 90% of the population, and in the end, defiled the Sunwell itself by using it to resurrect the evil necromancer Kel'Thuzad.
Kael'thas Sunstrider, descendent of Dath'remar Sunstrider, was out of town at the time of the Scourge invasion, but he came back to try and salvage his kingdom as best he could. He found scattered survivors hiding here and there, all weak and ill, with torn spirits and withered hopes. To them he seemed like a savior, the only one with wisdom and vision to lead them out of their despair. In honor of the fallen, he suggested his people adopt a new name, "blood elves," so that this tragedy may never be forgotten.
Kael'thas was forced to destroy the corrupted Sunwell in order to purge the blood elves of the undead taint now corrupting it (without knowing at the time that some of its energies had been preserved). Now that this energy was lost, however, he and his people had to look to new sources of power to feed their addiction to magic. The Alliance, however, completely failed to understand the true nature of the blood elve's plight. Grand Marshal Garithos, who had appointed himself leader of the Alliance forces remaining in Lordaeron after the Scourge had come through and destroyed nearly everything, believed that humans were superior to all the other races, and wished the elves and dwarves had never been allowed to join the Alliance. He gave the blood elves orders which were at first menial and then simply impossible to achieve. Garithos set them to defend themselves against an army of the undead, and then abandoned them at the last moment, withdrawing all the human forces into a different battle. Kael'thas and his regiment were forced to accept the assistance of the snake-like naga in order to avoid certain destruction. Garithos became enraged that the blood elves had accepted the help of their long-time enemies, despite the fact that he had left them with no other option, and so all relations between the blood elves and the Alliance broke down completely. Lady Vashj, leader of the naga, brought them to Illidan Stormrage in Outland, who taught them how to use demonic energies in order to meet their urgent need for magical energies.
A new hope
Nonetheless, to the blood elves remaining in Azeroth, it seemed like a new day was dawning. Kael'thas seemed every bit the great hero his ancestor Dath'remar had been (or seemed to be), and few suspected that even greater dangers lay in wait for them. Many of the most capable of the blood elves went with Kael'thas to Outland, while the remainder stayed behind to rebuild as much of Quel'thalas as they could. These blood elves still in Azeroth looked forward to the day that they could rejoin their leader in Outland, but in the meantime forged ties with their former friends among the Forsaken of Sylvanas Windrunner, and by extension, with her associates in the Horde. It seemed a temporary alliance of convenience at the time, but in truth it was the last refuge of their teetering culture, about to withstand yet another psychological blow, when their king and hero would become their enemy.
This brings us to everything your character would have known at the outset of the Burning Crusade, when blood elves were first introduced into World of Warcraft. More than any other race in the game, however, the story and conditions of the blood elves has changed a great deal since the Burning Crusade was first released. Next week, we will have a look at these developments and see how the blood elves stand as the attention turns toward Northrend, and their ultimate confrontation with their greatest nemesis: Arthas.
For more information about the blood elves, feel free to check out WoWWiki's information about them, but beware that reading it feels a bit disjointed, since the story of the blood elves actually branches off from the stories of both the night elves and the high elves, both of which have retained their identity as separate groups (although blood elves now far outnumber the remaining high elves who rejected the changes that Kael'thas brought and remained loyal to the Alliance). Dramatis-Personae, as usual, has a good basic introduction to blood elves. Together with this article, these should give you everything you need to get started roleplaying as a low-level blood elf, but as you level up your character will need to learn more and more about the changing state of their race...
...to be continued.
[Edit: I have updated the "new catastrophe" section to reflect my deeper understanding about how the blood elves were betrayed by, and ultimately came to sever ties with the Alliance. Thanks to all the commenters for pointing me in the right direction!]