Harry could be curious. He might immediately start to wonder what Blaise has to say, especially if Blaise has never reached out to him before. So he'd be eager to open the letter and find out the contents.
If Harry has had some positive interactions with Blaise prior to the letter in the fanfic, he could be happy and excited. He might view it as an opportunity to strengthen their relationship, or perhaps he's been hoping for Blaise to make this kind of move so they can work together on something, like solving a mystery in the school or dealing with a common enemy.
He might be suspicious at first. Given their different houses and the general tensions between Slytherin and Gryffindor, Harry would be cautious about Blaise's motives.
Perhaps it's a warning. There might be some danger approaching Harry, and Blaise, having come across this information, decides to inform him. For example, if there's a new dark wizard in the school or some sort of plot against Harry that Blaise has uncovered.
Ginny could be a bit shy about it. She might read the letter over and over again in private, blushing each time. Then she would write a response letter, expressing her own feelings for Harry in a more reserved but still very loving way.
Blaise might be horrified and disgusted. He would probably immediately stop reading it.
If the letter contains bad news or a problem, Hermione would likely be very concerned. She's a problem - solver, so she'd start thinking about solutions right away. For example, if Harry tells her about a new danger in the wizarding world, she'd start making plans to protect themselves and others. Her reaction would be one of determination and focus.
Maybe the love letter contains his deep admiration for Ginny's bravery. He could mention how she always stands up for what she believes in, whether it's against the Dark Arts or for the rights of house - elves. He might also share some of his insecurities and how Ginny makes him feel more confident, like when she supported him during his difficult times fighting Voldemort.
One popular 'Blaise and Harry boyfriends fanfic' is 'The Unlikely Bond'. It has a great plot where Blaise and Harry start off as rivals but gradually develop a deep emotional connection. The author does an amazing job of portraying their relationship growth, from hesitant friendship to a loving relationship.
Maybe Blaise sees something special in Harry. In fanfiction, it could be that Blaise admires Harry's courage or his unique abilities. He might feel a sense of kinship with Harry despite their differences.
Some students might be shocked. For example, Ron would probably be open - mouthed in disbelief. Hermione might be initially scandalized but then try to analyze the situation rationally. The Slytherins could have a mix of reactions. Some might be disgusted as they don't expect Blaise to be involved with Harry. Others might see it as an opportunity to tease Blaise or get some leverage over Harry.
Harry would likely be stunned. His world has been turned upside down. He might initially be in denial, thinking it's some sort of cruel joke or a mistake. But as he starts to accept it, he may feel a new sense of connection to Blaise. He could start to see Blaise in a whole new light, noticing similarities in their personalities or abilities that he hadn't before. And he would probably be eager to learn more about their shared heritage and what it means for their future together.
Another theme might be the reaction of the wizarding world to such an unusual situation. It can be about how society views their relationship and the pregnancy, and how Blaise and Harry navigate through the potential prejudices and challenges. In many fanfics, this can lead to some interesting plot developments as they try to protect their family and each other.