Been doing this with gZoom.. I get about 4 keys per hour
How to farm Black Lion Keys:
Create a new character, choose a Warrior Human from the Common folk and rush your way to the level 10 story quests that rewards you with a Black Lion Key.
Don't do anything else than the story quests, when you get to choose between orphanage or hospital, choose hospital.
A run is around 25 minutes. Can be done with other people to speed up the process.
Guild Wars 2 - How to farm Black Lion Keys - YouTube