Not saying it's the right way to go about it but the idea is more for evening out the power balance in the relationship. Initially, he's the only one who had something on her so he was in a position where he could blackmail her if he wanted. Now it's mutual.
No, it's not. He has said he worn't tell a soul but she took the photo (which he was framed for) AND told him that were she to spread it (which include lying, or at least untruthful testimony), he'll be ruined. If she needed proof of that, she would have known he would hace ruined her anytime he wanted, and every time they do anything is more material for hi to bury her deeper. It wasn't about equality, it was about getting the upper position because she knew he would chicken out.
Rather, calling it blackmail with either is a bit of a disservice.
She told him specifically "If I were to spread this photo it'd ruin you". Yes, that's considered legally and morally blackmail, or at the very least, threats.
Relationships are kinda like this. Especially as you explore eachother's kinks and eccentricities, you end with a lot of 'dirt' on eachother. You just trust they won't tell anyone...mostly because you got some embarrassing shit on them too.
That's depends of the person. When I tell something like that, it's because I trust them with the info, I don't care if it's known. If they tell, I don't care.
Actually, ya'll probably do this shit with your friends too. When you start talking about your fetishes with eachother or when you saw them do some really dumb shit. A big part of relationships is the secrets and sus moments kept between the two.
No, that's actually a confidence issue. If any friend pulled shit like she did, breaking the friendship would be THE LEAST of their problems. Sharing secrets it's fine, blackmail it's not.