I’ve found the fastest way to get a Black Lion Key is to create a new Human Warrior and do the commoner story mission. You can finish the level 10 mission at a lower level by letting the NPCs do a lot of killing for you.
Is it an exploit to do this repeatedly to collect Black Lion Keys? I don’t want to be banned, and I don’t know if you would consider this “too good to be true” or not, since it takes at least thirty minutes of effort, but generates a hard to find item.

What I was told was that the devs felt that the time spent was acceptable in relation to the rewards. John later confirmed that it wasn’t an exploit, so I believe the question is covered now.