I feel like the blackmail tag us kinda unnecessary... I mean the blackmail is really only in the first few chapters.

Literally said that they'd stop being friends if she didn't listen to her requests in this chapter. Coercion, manipulation, blackmail, whatever you wanna call it.

You didn't understand the meaning why she said that. Ever since that talk in the nurse office Sora figured out there is a chance their feelings is mutual but then Akane request that she want to be friends and it shows how dense she is about romance that why she laugh and agree to be friends. Saying she will stop being friends and telling Akane that they are friends when she didn't kiss her is for Akane to rethink their relationship as something more. They are both have unfair motive from the beginning and Akane admit that she agree to all request even without that recommendation. It could only be called blackmail if she is doing it against her will but now she is just willing to fulfill all request.