Historically speaking the idea of a Dark Jedi is tantamount to a fallen Jedi. Which is to say, a character trained in the traditions of the Jedi who, for whatever reason, has broken from the Code and Order as a result, cast out. The idea being that if you're a Dark Jedi, the Jedi part still rings true.
There are a myriad variety of alternate disciplines, however; and I encourage you to continue your research. One need not conform to any formal method of practicing use of the Force. For instance; it is believed that before the formal solidification of the Jedi Order some tens of thousands of years prior the Galaxy was rife with a number of disciplines. And this continues to this day - one need not look far to see examples of this.
Do not let yourself be caged in by what you perceive to be relevant.
