Hermione would probably be angry at being put in such a situation. She might try to enlist the help of her friends, like Harry and Ron, in a very secretive way. She would be careful not to expose the blackmail completely because she doesn't know how Snape would react if he found out she was trying to get help. She could also use her vast knowledge of magic to create some sort of protection or counter - measure against Snape.
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.
Hermione usually reacts with a mix of anger and determination. She's not the type to be easily pushed around. So, she might start by trying to figure out who the blackmailer is and what their motives are. For example, she could use her intelligence to set traps or follow leads to expose the blackmailer.
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.
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.
Snape might initially be taken aback. He's used to students being intimidated by him, so Hermione's outburst could shock him into momentary silence. Then he might retaliate with his usual sarcasm, trying to put her in her place.
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.
Snape might initially be taken aback. He's not used to students standing up to him so forcefully. He could glare at Hermione with his cold, dark eyes, trying to intimidate her into silence.
One possible plot could be that Snape discovers Hermione has been sneaking into the restricted section of the library for some research that could get her in big trouble. He threatens to expose her to the school authorities unless she does his bidding, like helping him with some dangerous potions experiments. This would put Hermione in a difficult situation as she doesn't want to break the rules further but also doesn't want to get in trouble.
He might be initially suspicious. Snape is a complex character. He's used to being on his guard, especially with Gryffindors like Hermione. So, his first reaction could be to wonder what Hermione's real motives are for seeking his help.
Snape would probably be very stern with Draco. He might give him a severe scolding and deduct points from Slytherin. Snape is a strict teacher and he won't tolerate such behavior from his own house students.
She might be initially defensive. Hermione is a proud and independent character. So, when Snape finds out about her arm, she could be worried that he'll use it against her in some way, like taking away house points or using it as an excuse to give her extra work.