Eu o termo "Dark Jedi" como entendido agora pode ser um oxímoro, mas a intenção original do termo era descrever os jedi que haviam caído. O termo Sith ainda não havia sido inventado. Da Wikipedia:
Dark Jedi and the Sith:
Dark Jedi is the unofficial name given in the Star Wars universe to antihero fictional characters attuned to the Force and adept in its dark side. The concept of "Dark Jedi" is not endorsed anywhere within the movie trilogies. They exist by that name only in the Star Wars expanded universe, including video games such as Star Wars: Knights of the Old Republic and the Star Wars: Jedi Knight series; the term is never used in any of the six Star Wars films. So called Grey Jedi and Rogue Jedi are terms that refer to Jedi who don't follow the Jedi Code and/or the decisions of the Jedi Council, or who have defected from the Jedi Order and/or have rejected the Jedi Philosophical concepts of "Lightside" versus "Darkside". Fallen Jedi refers to former-Jedi-turned-Sith who were seduced by the Dark Side (such as Revan or Anakin Skywalker).
Definitions:
Dark Jedi and Grey Jedi are a label to a specific archetype of characters in the Star Wars Expanded Universe defined as Force-attuned beings that meet two requirements:
They do not hold themselves to the dogma of either the Jedi or the Sith.
They use their Force abilities for less than virtuous pursuits–acts that are normally forbidden by the Jedi Order.
Dark Jedi in the Expanded Universe typically refer to pupils of Luke Skywalker's Jedi Academy who have fallen victim to the dark side. Dark Jedi are also extremely common in the Knights of the Old Republic series in league with the Sith Empire and are often fallen Jedi from the Mandalorian Wars. Dark Jedi also appear in the Jedi Knight series of Star Wars video games, where Kyle Katarn uncovers Imperial plots linked to Dark Jedi. In Jedi Outcast, Katarn faces an entire army of Dark Jedi, cloned and lightly trained force users who follow the main antagonist. Dark Jedi have distanced from Sith in more modern Star Wars media. They are not sworn enemies of the Jedi Order and do not consider destroying it a priority as Sith do. Dark Jedi do not always appear in pairs with a master and apprentice and often work alone.
Because the term Sith was never spoken in the original trilogy (although Darth Vader was described as "Lord of the Sith" in the published screenplay), early Expanded Universe products usually considered the "evil Jedi," those who joined the dark side of the Force, as "Dark Jedi." In his novel series The Thrawn Trilogy, author Timothy Zahn labeled Sith Lord Darth Vader and Emperor Palpatine as Dark Jedi, and the term "Sith" was never mentioned in the series until later reprints of the novels.
Portanto, era basicamente um termo genérico usado para descrever os usuários de força que não necessariamente seguiam os dogmas Jedi ou os "Sith", e geralmente usavam a força por menos que boas ações.
Quanto à atual Canon da Disney
Aqui está a definição de um "Dark Jedi" de Leland Chee, o guardião oficial da continuidade de Star Wars:
A dark Jedi is anyone who practices the dark side of the Force whether or not they were a former Jedi.
from moviepilot.com/posts/3652640
Quanto à sua pergunta exata
So is there any level of canon where there are any Jedi that would actually fit this definition of a "Dark Jedi" - one who believes in the Jedi Code, but intentionally goes against it?
Eu não acredito que haja. Existem exemplos de Jedi Sith ou Dark infiltrando a ordem por várias razões, tanto pessoais quanto em uma tentativa de derrubar os Jedi e a república, mas nenhum exemplo eu poderia encontrar de um Jedi, decidindo que eles não queriam mais seguir o Código Jedi, mas permanecendo na Ordem Jedi, e agora usando o lado negro da força.
Quanto a Quinlan Vos como exemplo
De acordo com wookiepedia, (que ainda é baseado no cânon do Legends neste momento)
He actually became a member of the Sith when he infiltrated the Jedi Order in an attempt to get his Revenge, so he doesnt even technically qualify.
Eu não estou ciente de sua história sobre o Disney canon o suficiente para falar se isso mudou ou não.