There are quite a few out there. You can start by looking on fanfic platforms like Archive of Our Own. Some might focus on an alternate universe where Captain Marvel has a romantic relationship with another female character, exploring different aspects of their relationship and how it might develop in various scenarios.
There are a few. One notable character is Wiccan from the Young Avengers, who is gay. And there are others whose sexual orientation has been explored in the comics.
One possible plotline could be the Black Cat character forming a relationship with another female character in the Marvel universe. They might start as allies in some sort of adventure and gradually develop romantic feelings as they face challenges together and discover more about each other.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfics across various fandoms, including many for Captain Marvel. Just use the search function and look for relevant tags like 'Captain Marvel' and 'lesbian relationship' to narrow down your search.
I'm not sure about specific 'black cat lesbian marvel fanfiction' off the top of my head. But you can try searching on fanfiction websites like Archive of Our Own. They have a large collection of Marvel - related fanfictions and you might be able to find what you're looking for there.
The Avengers originated in Marvel Comics. Marvel has developed a vast universe of characters and storylines, and the Avengers are a key part of it. Their adventures have been adapted into various forms of media, including movies and TV shows.