Being around fel energy causes corruption. You do not have to actively utilize it. See: Frostwolf Clan, Broken, and
"Even so, the prince's relatively quick acceptance of dire measures (e.g., draining magic from demons) is by no means characteristic of blood elves in general. The blood elves of Outland have by now discovered Kael'thas' agreement with Illidan, and they have for the most part become convinced of its necessity. Most blood elves still live on Azeroth, though. Few of these elves know of Kael'thas' pact with Illidan, and many would be horrified if they discovered it. Draining magic from small mana-bearing vermin is a far cry from draining magic from demons. Yet, as their hunger grows, blood elves--particularly those in Outland--are becoming increasingly inured to the things they must do in order to obtain more magic"