Kael’thas and surviving high elves become blood elves

Full History

After the Scourge devastated Quel'Thalas, Prince Kael'thas named the surviving high elves the blood elves in memory of their dead. The change of name answered a national disaster: the slaughter of their people and the destruction of the Sunwell around which their society had long depended. The survivors inherited the kingdom's grief and the problem of how to continue without its central source of magic.

Kael returned to a shattered homeland and gathered followers whose old identity no longer seemed to express what had happened to them. The name sin'dorei bound memory and survival together. Their arcane needs remained urgent, and the circumstances of the Third War forced them into service under human commanders who did not treat them as trusted equals. The prince's next choices would take him away from the remnants of Lordaeron's Alliance.

The blood elves' origin therefore lies before their later travels through the shattered remains of Draenor. Their change was driven by the Scourge's assault on Quel'Thalas, not simply by a new uniform or political alliance. Kael'thas's decision tied the name of his people to the loss of the Sunwell and set the stage for his alliance with Lady Vashj and Illidan during the Frozen Throne campaign.