PITTSBURGH — He didn’t blame the wide receivers for running incorrect pass routes, even if one of them seemed to be singling him out. He didn’t blame the special teams for allowing a third costly kickoff return touchdown in four games. He didn’t argue with offensive coordinator Bruce Arians’ decision to take over the play-calling during a hurry-up situation late in the game, when Ben Roethlisberger often is most effective while calling his own plays. No, Roethlisberger says the Steelers’ biggest loss of the season was his fault, and his alone. The Steelers (6-3) are down a game to the Cincinnati Bengals (7-2) in the AFC North after losing to Cincinnati 18-12, and the quarterback said the reason was No. 7. ‘‘You know what? I had a bad day. Everyone in here’s had a bad day. ... I flat out had a bad day and it’s going to happen sometimes,’’ Roethlisberger said Thursday, the first time he’s talked to reporters since Sunda.