I've read my share of 'Black Butler' fics with Sebastian and a reader-insert, and honestly, a lot of them don't really dig into the dark romance angle as much as they could. They'll give you the aesthetic—the Victorian setting, the fancy clothes, the polite menace—but then the relationship dynamic ends up feeling pretty sanitized. Sebastian's a demon who literally owns Ciel's soul; that's the core of his character. A dark romance with him shouldn't just be about him being protectively possessive in a cute way. It's about the inherent imbalance, the constant negotiation (or lack thereof) around consent when one party isn't human and has fundamentally alien desires. Is the reader character another contract, or something else? The best fics I've seen play with that horror. There's one where the reader is slowly corrupted, not by evil acts, but by Sebastian introducing them to increasingly refined, unbearable pleasures—sensory experiences so intense they begin to erode their humanity, making them dependent on him for that next fix. That's dark romance: the appeal is the danger, the corruption, the thrill of being with a creature who might adore you and also consume you, and the line between those things is vanishingly thin. A lot of stories skip to the comfort part without earning the dread, which misses the point.
What makes it compelling, when done right, is the push and pull of power. Sebastian is eternally bound by his nature and his contract. A reader-insert introduces a wild card. Does he view them as a fascinating toy, a potential snack, or something that genuinely, unnervingly complicates his existence? The romance isn't in him being tamed; it's in him choosing, for his own inscrutable reasons, to engage in a dance where the human partner is always one misstep from disaster. The tension comes from that precarious balance, not from will-they-won't-they arguments. It's the slow realization that the gilded cage you're in has no door, and you're not sure you want one anymore. That kind of story requires a writer comfortable with moral ambiguity and psychological unease, not just candlelit dinners and demonic cuddles.