In Star Wars canon, can there be Jedi who use the dark side of the Force and are not Sith?
Yes. The article on the Dark Side of the Force on wookiepedia lists several organizations that exist in canon as users of the dark side:
Dark Jedi: Probably the most confusing group, Dark Jedi either were former Jedi who chose to follow the dark side or those with no light side training that simply chose the dark side. The difference between them and the Sith were the specific teachings of the Sith, however they occasionally worked along with the Sith.
Nightsisters, or Witches of Dathromir: The best-known group of dark side users outside the Sith themselves. The dark side users of a shamanistic culture, they led the matriarchal tribes of Dathromir. Noted for riding rancors.
Rakatan Infinite Empire: The first known galactic government, they enslaved billions and used dark side powers to fuel their war machines and to create more with the Star Forge.
Prophets of the Dark Side: Technically a splinter faction of the Sith, this cult was founded by a renegade Sith Lord and eventually rose to some prominence under Emperor Palpatine.
Bando Gora: A dark-side worshiping criminal organization, singled out as a threat by both the Republic and the Sith.
Sorcerers of Tund: Reclusive group of dark side mages native to Tund. Rarely ventured off-planet.
Edit: In the Legends novel Darth Bane: Dynasty of Evil, I found the following passage (p.68 of the hardcover edition). This provides an Old Republic Jedi's perspective on the difference.
"What's the difference between a Sith and a Dark Jedi?" Lucia asked.
The Ithorian stopped pacing and turned to face them, instinctively addressing his audience like a teacher giving a lesson.
"The Sith were the sworn enemies of the Jedi and the Republic. They sought to wipe us from existence; they sought to rule the galaxy. They united their strength in the Brotherhood of Darkness, drawing countless followers to their cause with false promises. They amassed an army of individuals foolish and desperate enough to believe their lies, and they plunge the galaxy into a war that threatened to destroy us all."
"A Dark Jedi, on the other hand, has much smaller ambitions. He - or she - thinks only of himself. He acts alone. The ultimate goal is not galactic conquest, but personal wealth and importance. Like a common thug or criminal, he revels in cruelty and selfishness. He preys upon the weak and vulnerable, spreading misery and suffering wherever he goes."
These Force users are referred to by the moniker of “Dark Jedi.”
Among the possible Dark Jedi in canon are:
She’s explicitly referred to as a Dark Jedi in Star Wars Insider #155, the first appearance of this term in Disney canon.
It wasn’t until season one of Star Wars: The Clone Wars that we’d see an all-female lightsaber duel. And what a duel with was, with Jedi Master Luminari Unduli and Padawan Ahsoka Tano taking on the Dark Jedi Asajj Ventress. At first, Unduli believes she can defeat Ventress easily, due to the Dark Jedi’s undisciplined fighting style—an over-confidence that is not very Jedi-like.
She fell to the dark side before being taken on by Dooku. At some point, though, she definitely became Dooku’s Sith Apprentice, not merely a Dark Jedi assassin, which motivated Sidious to order Dooku to kill her. Certainly the Jedi (and the narrator) refer to her as a Sith throughout Dark Disciple.
Asajj Ventress had indeed once been Count Dooku’s Sith apprentice—and pet assassin. Kenobi and Anakin had crossed lightsabers with her on more than one occasion.
On the other hand, she’s described in Star Wars: Absolutely Everything You Need to Know as "not fully trained in the ways of the Sith," so it’s not clear which term better applies to her during the Clone Wars series.
In any case, while the label "Dark Jedi" might still have been technically accurate for her after Dooku starting training her as an apprentice (which may have happened before her fight with Unduli), it was the only term that applied both before Dooku took her in and after she left him.
He went over to the dark side for a while. It’s not clear whether he was ever a Sith. Dooku implies that he was:
“I won’t be your apprentice!” Vos started to choke Dooku again.
“No, no!” Dooku wheezed. “A team. Equals.”
“Sith don’t work that way.”
“Are you and I ordinary Sith?”
Vos still looked skeptical. “You know where he is? And do not lie to me!”
The Grand Inquisitor
He was a former Jedi Temple Guard turned evil. Some of the other Inquisitors may have been fomer Temple Guards or other Jedi as well.
His ambition was to become Dooku’s apprentice, but he never got the chance. He definitely fell to the dark side, though.
She may not have been a Dark Jedi. It’s possible, though.
A dubious example. He was part of Luke’s new Jedi academy, then turned to the Dark Side. He’s not a Sith, but he’s part of a formal dark side organization (the Knights of Ren).
This doesn’t count the really temporary examples of Anakin and Ahsoka on Mortis, which may have been a Force vision anyway, or of Count Dooku, who may have fallen to the Dark Side before Sidious sought him out.
It’s worth noting that all Dark Jedi in canon are either former Jedi, or concealing their affiliation. Employing the Dark Side is something the Jedi Order frowns upon very deeply.
An important note is that there are non-Sith Force users who draw upon the Dark Side, but are not Dark Jedi, since they were never Jedi. Among these are, for example, some members of the Nightsisters, such as Mother Talzin.