Leah stated that it was Nemesis who altered Victor Sells, and the FBI agents. But wasn't that just black magic? Doesn't a wizard go insane as they use enough of it? The Black Staff pulls it from the wizard when casting as it did with Eb. Harry was exposed to it just as the FBI agents were when he used the wolf belt.
Here is my guess. Just as love is the stuff of Creation, I'm thinking that dark magic is the stuff of hate or Anti-creation, Nemesis. Just as the Archangel's have used their Knights as a trumpet (Murphy in Changes), Nemesis can speak to, or through anyone who's been contaminated. It's why Leah, and the other Sidhe had to be infected. They don't use mortal wizard magic, and cannot become contaminated that way. Wizards on the other hand can pull it within themselves, or transfer it to objects like the Hexen Wolf belts. I'm thinking that Leah attempted to access the power of the blade, and that's when it contaminated her.
What I'm curious about is why mortals seem longer to go dark side from using dark magic vs how easily the Sidhe fall once exposed to it's influence.