A load of fics have a past Seblaine relationship. Here are the AO3 search results for completed stories tagged with Blaine Anderson/Kurt Hummel and Blaine Anderson/Sebastian Smythe. Make sure you read the tags and summary carefully to make sure that Klaine is the end game. -  HKVoyage

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Independent Living by @gleekmom

AU Klaine Fic: After the death of both his parents, Kurt Hummel has been living in foster care for the past 10 years. Now he's 18 and has finally achieved his goal of Independent Living. But when he meets Blaine Anderson, he finally learns that being independent doesn't necessarily mean being alone. (Warning: Seblaine History)

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Love Means Nothing (in Tennis) by fallovermelikestars

Wimbledon is arguably the most prestigious tennis tournament in the world and Blaine Anderson, once America’s darling, is now only here as the wildcard. Blaine has one goal - to play tennis and then, win or lose [and his money is on lose] to retire. He doesn’t bank on Kurt Hummel, young, handsome and with a backhand that would make even Djocovic weak at the knees. The rising star. As they battle against each other for that coveted trophy, Blaine finds himself wondering: is love actually more than a zero score?

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Take It Like a Man by idoltina, penguinutopia

A Legally Blonde AU. Blaine Anderson has it all. He's a member of both the polo and fencing clubs, founder of the charity Songbirds in Schools, and the lead soloist of the university’s famous all-male a capella choir, the Warblers. In his senior year at UCLA, Blaine -- a show choir major -- wants nothing more than to be the future Mr. Blaine Anderson-Smythe. When Sebastian breaks up with Blaine instead of proposing and heads off to a more ‘serious’ life at Harvard Law, Blaine sets his sights on following the love of his life to Boston to win him back. But Harvard holds a lot more obstacles than Blaine anticipated: challenging coursework, Santana Lopez, and an admittedly cute graduate by the name of Kurt Hummel.

Note: Part 1 of Find My Way series

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How Things Should Be by Imogen_LeFay

Kurt Hummel's life is comfortable for a peasant with his work in the kitchen of Earl Anderson. It would be more comfortable, if he could rein in his feelings for the Earl's younger son, Blaine. But Kurt knows, life isn't a fairy tale. And lords marry ladies, they don't fall in love with kitchen servants.

Kurt and Blaine Wake Up Married by @seeroftodayandtomorrow

A drunk marriage in Vegas, a divorce temporarily out of the question and his new husband someone he's been a fan of forever: Blaine Anderson's week just got a lot more exciting. If only he wasn't still so hung up on his ex...

Inspired by the 'Wake up married' -series by Leta Blake and Alice Griffiths

The Glee Holiday by AnnParker

I’m pretty sure that in every Klaine fic where Kurt sings, people praise his angelic voice. Here are some fics that feature singer!Kurt.

Fanfare by Phantom of a Rose 

Kurt Hummel is everything Blaine wants to be: out and proud, talented, and courageous. Blaine would love to tell him how he feels, but then, Kurt Hummel is a famous singer and actor and he will never know that Blaine even exists. Right?

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Carry That Weight by hedgerose, penguinutopia 

When Blaine's friend Susan drags him to an American Idol audition, he's not even trying to get past the first round-- much less into the top ten. And after getting through Disco Week, avoiding the media, and dealing with his very absent parents, Blaine's not sure he even wants to win-- although falling in love might make up for a lot of that. And maybe, just maybe, he can win this thing.

Note: This is the first part of a series.

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Don’t You Want Me by orphan_account

Kurt is the flagship of Blaine and Cooper's label. When Kurt auditions for a role on Broadway, Cooper is so scared to lose him that he asks Blaine to seduce their star to make him stay.

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High Holidays by Twitchy Squirrel

In this AU Kurt is a thirty-something struggling opera singer. Blaine is an international pop superstar. They meet on a flight to Italy, and the rest is amore.

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The Bodyguard by VisionImpossible

AU. As a world-famous singer, Kurt Hummel becomes the target of a mysterious stalker. But on the other hand, he somehow just can't get his new bodyguard off his mind..

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Happy Reading!

This is a pretty popular premise. Pro tip: You can search on AO3 with more than one relationship. Blaine Anderson/Kurt Hummel and Blaine Anderson/Sebastian Smythe with the completed status produced 165 results. 

Under the cut, there are ten fic recs to get you started.

- HKVoyage