Inside their capital city of Silvermoon, in the Court of the Sun, you can find a Naaru (ethereal being of Light, there's lots of them in Shattrath, too) being held captive by the Blood Elves. This is how they're using the Light (to become Paladins and generally wreck havoc with Warcraft's lore) for their own purposes, by holding the Naaru named M'uru captive and draining his power.
But Naaru are pretty darned powerful (even Khadgar, if you talk to him, sounds impressed with what they've got), and so a few players, Arlia of Bloodscalp included, hypothesize that M'uru might not be completely on the level about being "captured." Maybe he's just playing possum-- though for what purpose we're not sure. It could be to study the Blood Elves, or to bring them to the Light eventually. Or there's the possibility that "M'uru" is not a Naaru at all, and the Blood Elves don't have exactly what they think they have.
Though it's true we can't know for sure from the lore available now, a few players say that what we see is real: the Blood Elves used to be the High Elves, and are just "magic badasses," capable of draining whoever they want, especially a magical form like the Naaru. But it would be very interesting if this Blood Elf-Naaru relationship was not what it seems.