Well, the Blood Elves haven't technically made that infamous blood pact Ner'Zhul originally did to infect the entire race with the chaotic mentality it once had. They are simply obsessed with the high magic which is synonomous with that context, and now driven by the notion of vengeance as well.
So, you could very well be right. Our Blood Mage campaign hero could turn out to be something of a villain as well, gradually consumed within the path of chaos. This could be Blizzard's way of delivering the whole race into suicide, or at least to a place where they stand as an enemy to all, and therefore are out of the nominees list for the WoW PC roster.
Then again, if Blizzard wanted to do that, they could've just let them all die with the previous Undead invasion of Quel'thalas. Chocked them all up as dead or Scourge initiates now.
I think what Blizzard is trying to establish with the Blood Elves is that, while they are still a people capable of corruption (as they already were), they have valor in them as well, and a committment to safeguard themselves and the world from the true forces of evil. This makes them a potentially more passionate force for ideals of justice, sensitive to the wrongdoing behind the machinations of the Scourge and Legion. They could be a race of vigilantes, and I feel that in particular actually makes for excellent PC materila, in an MMORPG context.
Regardless of WoW, I do think Blizzard is just trying to give their original elf faction a new spin. They make the lore up as they go along, from game to game. The High Elves weren't High Elves until WCIII, when they needed to be High to disassociate them with the new Night Elves. But the Night Elves themselves were a move toward a different image of the elf in general, and so, short of eradicating them (like I mentioned), redefining the High Elves was just another way to keep them kicking in the contemporary vision of the Warcraft world.