i've heard of them, though i'd use them like the Lord and Lady. haven't heard of many people who call upon a darkness dragon, have a few spells, but the only dark dragon i've ever come across so far is a Chaos Dragon. they're big and all black.
I see it this way,wiccans believe that there are five elements,fire,air,earth,water and spirit(I'm not a wiccan,but some of their stuff makes sense to me.)I see light and darkness as both being half of the element spirit,that's just one way to think of it.