(most interesting man in the world commercial)
I don't always use duo darkness decks, but when I do, I choose Earth
The reason is:
I usually preferred darkness mono, but then I found a good synergy if you added a couple earthquakes to it =P it helped a lot. Complete quanta control. Kill the pillars, devourers to absorb quanta, and then there is no real need for healing, and it's better to just go straight for the win rather than healing because it's much faster. That's why you pick obsidian dragons over vampires
and add some steals for permanent control(maybe even for more pillar massacre!)
Welll I just explained my deck, but who cares =P that's the reason I like earth in a duo with darkness best =P I don't really make duo darkness decks with any other elements though >_>