I remember the old EU used the term "Dark Jedi" quite a bit, especially when the prequels made it clear there could only be two Sith at a time-I remember the Dark Horse comics and several games, for instance, had Dooku have a whole slew of various force-sensitive apprentices apart from just Ventress (and later Savage Opress and Quinlan Vos). The "Jedi knight" game also took this approach as well.
Although when the prequels wrapped they pretty much started using "Sith" again, especially the "Legacy" stuff which pretty much had the Sith throwaway the Bane order stuff