Stealing the Light
At this point it is important to draw a distinction between the blood elves who followed Kael'thas and the naga through the portal into Outland, and the blood elves who stayed behind in Quel'thalas and Silvermoon City. They were still one faction at this point, but a number of differences were starting to appear. For one, although the blood elves in Quel'thalas were drawing on fel energies just like their brethren in Outland, they certainly weren't surrounded by demons like Illidan and all his minions all the time, not to mention the vast energies of the evil Twisting Nether, which surrounded all of Outland. Thus, the blood elves in Outland were saturated to overflowing in magic and power, while the blood elves in Quel'thalas were still rather hungry for it.
Therefore, Kael'thas thought it wise to send the gift of this captured naaru, named M'uru, back to Silvermoon City, so that his people there could have more energy to help quench their magical thirst. Soon, however, the blood elves of Quel'thalas found a way to start using this power of the Light rather than merely feeding on it, casting spells and blessings in the same way that human, dwarven, and draenei paladins could -- while the other races drew on the Light through the power of their faith, the blood elves learned to control it as it flowed through M'uru.
The first blood elf to take up this path of corrupted paladinhood was Lady Liadrin, who then founded the order of Blood Knights that became infamous throughout Azeroth and Outland alike. Thrall and the other leaders of the Horde disagreed with the methods the Blood Knights had employed, but could not deny their strategic value on the battlefield.