It depends if there was more to the blackmail.
Obviously this is gross speculation, but my stab in the dark is that the blackmail photos were accompanied with a demand (blackmail usually needs some instructions otherwise the recipient doesn't know what the fuck they want).
It's clear Mel and Sarah were hoping that Chad was going to break up with Sage. The scene when Mel and Sarah spot Sage and the mc making out on the HOTs yard:
Sarah: "See? I told you that they're more than friends."
Sarah: "No one gives private lessons for free like that."
Mel: "Maybe she's just fucking him on the side?"
Sarah: "No. You hide a side-bitch, you don't kiss them passionately out in the open."
Sarah: "Not when you're in a relationship."
Mel: "..."
Sarah: "This will make it easier when Chad breaks up with her."
Mel: "I'm gonna go call him."
Sarah: "Easier...but Sage will still be furious."
Mel: "If things ever go south..."
Mel: "...you'll have my back, right?"
Sarah: "Don't I always?"
Mel: "I'll always have yours too."
Why do they think that Chad will break up with her? Because the blackmail photos told him to do it or else? Although it's a shitty thing to do to someone, what Chad is doing to Sage is even shittier. By forcing his hand and making him break up with her, everyone actually wins in the long run.
But Mel and Sarah are aware that it could go wrong: "If things ever go south... ...you'll have my back, right?"
And when they witness the actual breakup, Mel is obviously distraught. It's clear it hurts her to see Sage so upset, it's not like they don't like Sage. Hence the previous comment: "This will make it easier when Chad breaks up with her".
Eventually the breakup scene is too much for Mel, and she leaves, with Sarah following her:
Mel: "I can't..."
Sarah: "Shh... It's ok. It will be ok."
Neither of them wanna see Sage or Chad hurting, but it's for a "good cause". Their plan is working, even if it's currently unpleasant to witness.
Then in episode 8 there's more panicking from plotting duo:
Mel: "Ok, Sage definitely knows something!"
Mel: "You saw that look she gave me."
Sarah: "I'm pretty sure that look was meant for both of us."
Mel: "I can't deal with this anymore... We gotta tell someone."
Sarah: "Who the hell would you tell to make this any better?"
Mel: "If we talked to Sage now, we could give her our version of the story."
Sarah: "It's way too late for that. It would crush her. That we knew all this time and never told her."
Mel: "Don't you think it's better than her finding out another way?"
Sarah: "It looks like we'll get blamed no matter what we do."
Sarah: "We're gonna have to wait and see what happens."
Mel: "If she accuses us...what do we say?"
Sarah: "..."
Mel: "Do we tell her the truth, or do we deny it?"
Sarah: "We deny it."
Sarah: "If she thinks that we're sluts and she ends up hating us..."
Sarah: "Fine! I can live with that. It's just for less than a year; then, she'll be gone."
Sarah: "I'm not telling her the truth."
Sarah: "Not only because it would end up hurting us, but also..."
Sarah: "...if I were in her shoes, I'd prefer the lies."
What the two of them don't realise is that Sage doesn't suspect any of their plan, she just thinks Chad is fucking one of them.
) and MC is shown getting under her skin, no matter how you play him.
