Fanfiction.net is also a good place to search. There are many different categories and filters you can use. Although it may not be as organized as AO3 in some ways, it still has a vast number of fanfictions, and you might be able to find some great 'dark Annabeth' works if you spend some time exploring.
One popular 'dark Annabeth fanfiction' might be those that explore her facing her inner demons in a more extreme way. For example, a story where she has to deal with a powerful curse that not only affects her physically but also mentally, changing her relationships with her friends in unexpected ways.
One possible plotline could be a darker version of their quests. For example, instead of simply fighting monsters to save the world, they might be forced to make morally ambiguous choices. Maybe they have to sacrifice some innocent bystanders to defeat a greater evil. Another could be a power struggle between them where their darker sides come out. Percy could be more power - hungry and Annabeth more calculating in a way that challenges their friendship.
One possible fanfiction could be about Percy and Annabeth's struggle in Tartarus, where the darkness of that place starts to seep into their minds. Maybe Annabeth begins to have dark thoughts about using the power of Tartarus to defeat their enemies, and Percy has to pull her back from the brink. They face all kinds of terrifying monsters and challenges that test not only their physical strength but also their mental stability.
A plot could be set in a post - apocalyptic world where Percy and Annabeth are on opposite sides at first due to some misunderstanding. As the story progresses, they realize they need to work together to face the real threats in this dark new world.
The plot could involve them losing their way in Tartarus, both literally and metaphorically. They get separated from their usual support systems and have to rely solely on each other. As they journey deeper, the darkness starts to change them. For example, Annabeth might become more aggressive in her decision - making, which is a departure from her normal self. Percy struggles to keep up with this new side of her while also fighting off the demons of Tartarus.
In the fanfiction, Dark Percy might have a darker sense of humor, one that is more sarcastic and cutting compared to the original Percy's light - hearted humor. His powers could also be used in more menacing ways. As for Dark Annabeth, her leadership skills could turn into a more tyrannical form. She was a great leader in the original, but in the dark versions, she might lead with an iron fist, not caring about the opinions or feelings of those she leads, and being more focused on her own vision regardless of the consequences for others.
Percy could be developed as a more brooding and conflicted character. His powers might be darker and less controlled, which makes him a danger to those around him, including Annabeth. Annabeth, on the other hand, could become more determined and a bit more ruthless in her attempts to save Percy from his dark self.
Annabeth could also be angry, not at Percy but at the situation or the forces that led to his death. She might vow revenge and start on a journey to find out who or what was responsible for Percy's sacrifice and make them pay. This could lead her into a whole new set of adventures and challenges as she tries to come to terms with her loss.
Maybe an antagonist who wants to disrupt the group dynamic. If Annabeth is a key part of a group, punching her could cause chaos and confusion among her friends. It could also be someone who has been influenced by dark magic or some sort of mind - control in the fanfiction. This would make them act against their normal nature and punch Annabeth without really having a personal grudge.