In some fanfictions, Hermione will initially be shocked and a bit scared. But then she'll pull herself together and try to negotiate with Snape. She might offer him something else in return for him dropping the blackmail, like helping him with his research in a different way than he originally demanded.
Hermione may also start to scheme. She'll use her intelligence to come up with a plan. For instance, she might start to collect evidence against Snape, something that she could use to counter his blackmail. She'll research old school rules or find out secrets about Snape that she can hold over his head instead.
In some fanfictions, Snape might blackmail Hermione over her grades. For example, he could threaten to expose some small mistakes she made in her potions work if she doesn't do him a favor, like helping him organize his ingredients. This creates a tense situation between them and often leads to Hermione trying to outwit Snape in some clever ways.
She might initially be shocked and scared. But then she would start to think rationally and try to find a way to outwit Snape. For example, she could start to gather evidence against Snape's own wrongdoings, if there are any, so that she can turn the tables on him.
She often tries to ignore it at first. She's focused on her studies and doesn't want to give the bullies the satisfaction of a reaction.
Snape might initially be shocked. He's not one to show much emotion openly, but internally he could be very disturbed. He might start investigating what happened to Hermione immediately, using his skills in potions and magic to try and find a cure or at least understand the cause.
Hermione might initially be very suspicious. Given her intelligence and wariness, she would not easily fall for the temptation. She would likely question Snape's motives and try to analyze the situation from all angles.
Hermione would likely react with her characteristic intelligence and courage. She would probably try to use her vast knowledge of magic to find a way to escape or at least resist Fenrir. For example, she might try to use spells that could incapacitate him temporarily. She would also be likely to try and outwit him in conversation, looking for any weaknesses or loopholes in his plans.
Snape would probably be quite surprised at first. Hermione is known for her academic prowess and self - sufficiency. His reaction could range from being dismissive, thinking she should be able to solve her own problems, to being intrigued by the nature of her request. If it's something related to the dark arts or a mystery in the wizarding world, his interest might be piqued, but he'd still be guarded in his response. He might even try to use her request as a way to gain some advantage or information for himself.
Hermione would probably be very confused at first. Snape has always been a rather intimidating and strict teacher, so his sudden change in behavior would be quite a shock. She might initially think it's some sort of trick or test.
He could also be a bit sarcastic in his response. Snape has a sharp tongue, so he might say something like 'Miss Granger, what makes you think I have the time or inclination to help you?' But as Hermione explains her situation, he might gradually soften, realizing the seriousness of the matter. Maybe he starts to see a bit of himself in Hermione's thirst for knowledge.
Hermione often shows great courage in these situations. At first, she might be scared, but she soon overcomes it. She tries to gather information about her kidnappers and their plans. If they are trying to 'adopt' her, she questions their motives and tries to find out what they really want from her. She may use her intelligence to manipulate the situation to her advantage. For instance, she could pretend to go along with their adoption idea while secretly planning her escape. She also tries to stay true to her values. If her kidnappers have a different view on things like the treatment of Muggles, she will not be afraid to speak up and defend her beliefs, even in a captive situation.