Sith are not "Dark Jedi".
Sith follow a particular Code, while Jedi follow a different Code.
Dark Jedi are Jedi who have stopped following their Code.
I'm not sure there is an equivalent for the Sith. The Sith code is so selfish and individualistic that any behavior can be justified by it, even light side behaviour.
All Dark Siders are Dark Jedi, that include the Sith.