There do exist games without blackmail. The eWardrobe, for example, starts the MC with female clothing and then the MC is free (more or less) to choose whether he accepts becoming more girly or resists it as much as he can.
Lots of games use some form of coercion rather than outright blackmail. In Secretary the MC can choose how fast he feminizes, within the context that he really has no choice but to submit to his boss's will. Technically, it's not blackmail, per se, but the entire game has a LOT of coercion. In contrast to Perverted Education, the prime example of an MC being blackmailed. The Sissy Girlfriend Experiment also uses coercion rather than blackmail, as does SQ Cruise Ship. In The Making of a Slut the MC is blackmailed by multiple NPCs. At least in PE it's only the MC's teacher that blackmails him, though almost everyone else abuses him in one form or another.