Based on the story & clues so far I'm guessing the mystery blackmailer = Madeline
I'll explain why below
It may seem weird, as she is the mother of the 3 girls the MC is flirting, hitting on, getting into risque situations etc.
However she is also very kinky, we all know this as she gives her daughters S & M style "punishments".
Also the texts manner of language seems very firm, to the point, & straight up commanding which has been & matches Madeline's behavior as we've seen so far. Like if she does not get what she wants then MC is basically fired & who else can fire him then the one that hired him.
And even if this is not related to the story in any way, we have not had any real content with Madeline. I'm also guessing that it's being saved by the dev when MC discovers who the blackmailer is or who he thinks it may be & thus we get more Madeline content then. As this blackmailing story-line can't last too long (in my opinion).
I think its a riddle where the most obvious solution seems too easy to be true but it actually is the truth.