One common theme could be the struggle between good and evil within Lucy herself. Since she's using dark magic, she might have to grapple with the negative impacts it has on her moral compass.
There are many fanfiction platforms out there, but finding 'lucy uses dark magic fanfiction' might be a bit tricky. Some smaller, more niche fanfiction communities dedicated to the relevant fandom might be a good place to start. You could also ask in relevant forums or groups where fans gather to discuss and share such works.
There were many novel protagonists who used dark magic, such as the Harry Potter series, the Dark Tower series, and some characters in League of Legends. Dark magic is a mysterious power that can create terrifying visual effects and powerful powers, so it is used in many novels.
If the main character was in a novel using dark magic, then the story might involve the rise of dark forces, the revival of evil forces, and some mysterious magic and mysterious creatures. The protagonist might be a young man with powerful magical abilities. He/she might also face various challenges and difficulties that require him to constantly improve his abilities and eventually defeat the evil forces to protect the world from harm.
One popular 'Harry uses dark magic' fanfic could be 'The Dark Turn of Harry Potter'. In it, Harry might start using dark magic due to some extreme circumstances, like the threat of Voldemort being far greater than in the original story. Another could be 'Harry's Foray into the Dark Arts' where he explores dark magic for what he believes is the greater good, but it leads to a whole new set of challenges and moral dilemmas.
One popular star dress fanfiction could be 'Lucy's Starry Adventures'. In this fic, Lucy uses her star dress in really creative ways to battle villains. Another could be 'Star Dress Magic Unleashed' where the author explores different powers of the star dress that Lucy uses. There might also be 'Lucy and the Star Dress Realm' which dives into the origin of the star dress and how Lucy came to master using it.
In Fairy Tail fanfiction, Dark Lucy often represents a more complex and edgy version of the character. She might be portrayed as having a darker past or different powers. For example, she could have been influenced by some dark magic which changed her personality. This can add a new layer of depth to the Fairy Tail universe.
In fanfiction, Dark Lucy might be a part of a parallel universe story. In this universe, the events that shaped Lucy in the original Fairy Tail series happened differently. So she turned out to be 'Dark Lucy'. She could be a key figure in a war between light and dark forces in this alternate Fairy Tail world.
One possible fanfiction could be about Lucy using her celestial spirits in a really unique way during the Grand Magic Games. Maybe she forms a new and powerful combination of spirits that no one has ever seen before, giving her team an unexpected edge in the competition.
πI recommend the book 'Dark Magicians of the Heavens' to you! The main character of this novel, Xue Tian, was hit by a black magic book and began to practice black magic for a long time. The main character had an extremely black-bellied heart, which satisfied your need for the main character to know black magic. I hope you like this fairy's recommendation. Muah ~π