One common theme is loyalty. Blaise shows his loyalty to Harry in the face of danger or adversity. It could be protecting Harry from other students who might be bullying him or from more serious threats like dark wizards.
One common theme is friendship. Many fanfictions explore how these three characters, who start off with a lot of animosity, especially Harry and Draco, can form a friendship. Another theme is rivalry turned into cooperation.
Love against all odds is also a popular theme. Their different backgrounds and the expectations within the wizarding world make their relationship a struggle, but that's what makes it so appealing in the fanfictions.
One common theme is redemption. Often, Draco is shown on a path of redeeming himself and his relationship with Harry and Blaise plays a part in that journey. Another theme is friendship. They start from being enemies or at least indifferent to each other and develop a deep friendship over time.
One common theme is friendship. Despite coming from different houses and having different backgrounds, they learn to get along and support each other. Another theme could be mystery - solving. Given Hermione's inquisitive nature and Harry's knack for getting into strange situations, Blaise might be drawn into their adventures to solve mysteries around Hogwarts or in the wizarding world at large.
I'm not sure of specific 'Blaise protects Harry' fanfictions off the top of my head. But you can try searching on popular fanfiction websites like Archive of Our Own. Just type in 'Blaise protects Harry' in the search bar and you'll likely find some interesting stories.
Love is a very common theme. Harry's love for Ginny makes him want to protect her at all costs. Another theme is heroism. Harry is often depicted as the hero who steps up to keep Ginny out of harm's way.
One common theme is redemption. Draco is often seen as a character who needs to be redeemed, and Harry's protection plays a part in that. For example, Harry's actions can make Draco realize there's more to life than following the dark path.
A lot of these fanfictions deal with the theme of redemption. Maybe Blaise has a past she wants to overcome or Harry has some inner demons to face, and they help each other through it. Also, friendship turning into love is a common thread. They start as friends or acquaintances and gradually their relationship blossoms into something more. There's also the theme of power dynamics, especially considering Harry's status as 'The Boy Who Lived' and how that affects their relationship.
Another theme is the exploration of their magical abilities as a triad. In several works, they discover new spells or magical powers that are unique to their group. They might use these to fight against dark forces or solve mysteries. This is often an exciting aspect of the fanfiction as it adds an element of adventure. It also shows how their combined abilities are stronger than their individual ones.
One common theme is the idea of redemption. Blaise, perhaps having a past that's not so noble, saves Harry and in the process redeems himself. Another theme is the breaking of house barriers, as they are from different Hogwarts houses.