Get out of jail card.

I think the blackmail was more aimed at sending the pucture to his dad coz he put up a fight until the other guy threatened to send the pics to his father.

obviously, but it he was being blackmailed for being in a gay magazine so he theoretically could have called his bluff by revealing his abuser as gay himself by possessing said magazine. The jist of the blackmail was being possibly gay in a revealing magazine and having that be revealed to his father. Chiaki could have made others in the school aware of his blackmailers orientation because Japan his homophobic hence turning the tables, however it still remained his moles would have given him away so he'd still be at a standstill unless he somehow removed his moles.Regardless the blond ex is a piece of shit who honestly had no motivation for being a villain other than being written that way for the sake of the plot and a pitiable backstory for Chiaki.

You are working under the assumption that the ex cares about his reputation just as much as Chiaki cares about his father not knowing the truth. The blackmail works if only because Chiaki loves his father too much. I thought that much is clear, since Chiaki who seems to be perpetually chill, got really worked up at the mention of his father. The ex knew this and used it to his advantage.

As for the ex motivation, well, ask every single rapist in yaoi, and you most likely get the "I love him too much, so I rape him" scenario. What I find refreshing from this manga is Chiaki does not develop feeling for his ex, and does not end up with him, unlike the majority of yaoi out there.

You know I dont respond much because I cant form arguments or answers very well, but you just articulated perfectly what I thought but couldnt form