Dark Jedi=All Dark Siders, no matter they once were Jedi or not.
No, you've missed the point, to be Dark Jedi you have to be a member/former member of the Jedi order, this is part of the star wars canon
. Dark jedi have the benefits of techniques from both sides of the force, making them much more dangerous than other darksiders.
Some times non-force users describe Darksiders as Dark Jedi, this is akin to describing someone with a gun as a soldier, understandable but not something a professional would do.