You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created works, including crossover fanfics. Just use the search function and type in 'Blue Bloods Crossover' and you should be able to find relevant stories.
There could be a 'Blue Bloods Crossover Fanfic' where it crosses with a superhero show, say 'Arrow'. The police in 'Blue Bloods' might be dealing with a new type of criminal activity that is related to the super - villains in 'Arrow'. The cops could seek the help of the vigilante in 'Arrow' while also trying to uphold the law in their own way.
There could be a fanfic that delves into the backstory of one of the Reagan family members, like Erin. It could tell the story of how she decided to pursue a career in law and the challenges she faced along the way. This would give more depth to her character in the Blue Bloods universe.
One idea could be to focus on Frank's backstory before he became the Police Commissioner. Explore his early days in the police force, the challenges he faced, and the relationships he built. This can add depth to his character in the fanfic.
You could check out fanfic platforms like Archive of Our Own. There are often some great 'Blue Bloods Daughter' fanfics there. Just search for the relevant tags.
Good writing quality matters a great deal. This includes proper grammar, vivid descriptions, and a good flow. If the writing is choppy or full of errors, it's going to be hard for the reader to get immersed in the fanfic world of Blue Bloods.
The writing style and tone could make it distinct. If the author of 'blue bloods a perfect oc fanfic' has a more humorous or darker tone compared to other fanfic writers in the 'blue bloods' fandom. The way the story is told, whether it's through a series of flashbacks or a very linear narrative, can also set it apart from other fanfics. And perhaps the author has a different interpretation of the 'blue bloods' lore, which is reflected in how they develop the OC and the overall story.
A crossover fanfiction might have the law - enforcement aspects of both combined. For instance, the NYPD officers from Blue Bloods could learn from the unique ways that the Zootopia police deal with their diverse population of animals. Or perhaps an animal from Zootopia travels to the city in Blue Bloods and causes some chaos due to their different nature, and both sets of characters have to figure out how to handle it.
One popular crossover is with 'Law & Order'. The two shows have a similar setting in the law enforcement world, so it makes for an interesting blend of characters and storylines. Writers can explore how the Reagan family from Blue Bloods would interact with the characters from Law & Order.