What I thinks is: A Dark Jedi is not the same as Sith. When the Jedi first come out, there is no actually "Light" or "Dark", and after some "Rogue" Jedi become "Bad" and take out the Sith, they used the name as Opposite with the "Jedi".
So a Dark Jedi is a Jedi "Fall" in the wrond way, and a Sith is a person(Force-sensitive) who trained in the "Sith" way. A Dark Jedi have to learn the way of "Sith" to become a "Sith", so Mara is no a Sith, and of cource not a "Lord"
I am F-3PO human cyborg relation and I have been fluint in only two form of language.