For some odd reason, even though im a lv 58, which is the lv needed to train Blast of Discord, i simply cant learn it.
Even though it seems you have already figured this out, it may just be a Quest. I couldn't learn Grand Shadowdance, even though I was a high level enough for it. What I realized was, I had to do the Quest to get the first Power that Grand Shadowdance is off of: Shadowdance.
This may be what you're looking for; it may not. Even if it's not, I just wanted to help.