I don’t think it’s necessarily blackmail. I think Toujou was pointing out that they both are breaking the rules and they both could look out for one another to avoid punishment.
I think Akane dropped his cigarettes in front of their dorm door, and Toujou found them. That’s why he said, ‘then should I put these (back) in front of our room door?’
Toujou was pointing out that Akane is doing things that could get them both in trouble as well, so he shouldn’t be a hypocrite about the sneaking out.
Oh ! Wendy you say it like that I think you are right. I still hate how he went on about it though
*When
Well for now I don't like Toujou people who blackmail other people to get what they want.. urgh! The worst ! Still furious about where he is going at night though