Elemental Wands/staffs

Not by definition, no. That's something you made up by yourself. We don't even have opposing elements in MS since enemies are not of an element, just weak, neutral or strong to them. Opposing elements in MS could be fire-ice, holy-dark and lightning-poison even if the last one makes little sense. Not to mention dark element goes by unaffected by either boost or reduction.

In the case of elemental weapons, any element that is not increased by the weapon is considered opposing. Opposing is just the word used because it's a lot simpler than "element not used by the class". And so far the only dark element weapon is dark fog(correct me if needed); have we confirmed it isn't affected by elemental weapons?