There's a fanfic called 'Soul and Blackstar: Unlikely Allies'. This one focuses on how they start off as rivals but gradually become friends. It does a great job of portraying their individual strengths and weaknesses and how they complement each other as they face various challenges together.
There are also fanfictions that deal with the theme of rivalry. Soul and Blackstar have a bit of a competitive streak, and some stories play up this aspect, showing how their rivalry can be both a source of tension and a driving force for their personal growth.
The dynamic between Soul and Blackstar is what makes these fanfics appealing. They have such different personalities. Soul is more reserved while Blackstar is brash and confident. Fanfics can explore how they balance each other out.
One common theme could be friendship development between Soul and Blackstar with a touch of the 'lemon' aspect adding some romantic or more intense emotional elements.
One great place to start looking for Soul x Blackstar fanfiction is Archive of Our Own (AO3). There are many well - written stories there that explore their relationship in different ways, from action - packed adventures where they fight side by side to more emotional and character - study - like fics that delve into their feelings for each other.
One plot idea could be that Soul discovers Blackstar is pregnant and they have to deal with the shock together. They might struggle to keep it a secret from their friends at first. Another could be a story where an enemy targets Blackstar because of his pregnancy, seeing it as a weakness. A third idea is that the pregnancy causes Blackstar to develop new powers that he has to learn to control while Soul supports him throughout.
I'm not sure about specific 'Soul x Blackstar Lemon' fanfiction. You could try searching on fanfiction - specific websites like Archive of Our Own. There might be a good chance to find some interesting works there.
A more complex storyline could be set in an alternate universe where they are on opposite sides of a war. They have to battle each other, but deep down they don't really want to. This internal conflict, along with their loyalty to their respective sides, would create a lot of drama. As the story progresses, they might find a way to end the war without having to completely destroy each other.
Soul is often calm and collected. During the mpreg story, his calmness could be tested as he faces the unexpected situation. He might become more nurturing, showing a side of him that was hidden before. Blackstar, who is usually brash and confident, could become more vulnerable. His self - assurance might waver as he deals with the physical and emotional changes of pregnancy.
You can also check out Tumblr. There are often fanfiction rec blogs that specifically focus on Soul Eater pairings. They might recommend some hidden gems that are not as well - known on the bigger platforms. These could offer unique perspectives on Death the Kid and Blackstar's relationship, perhaps from a more emotional or psychological angle.