Perhaps she sees parallels between the story of Bluebeard and her relationship with Bill Callahan. Good 'ol badass, ladykilling Callahan has probably left a trail of women in his wake (in the same way that Bluebeard moved from wife to wife after he had disposed of the previous one), but Joanna was charmed by him and gave him a chance. Eventually, however, Joanna came to realise what had caused Bill's previous relationships to fail, his 'secret,' as Bluebeard's wife eventually discovered his terrible secret.
Perhaps she sees parallels between the story of Bluebeard and her relationship with Bill Callahan. Good 'ol badass, ladykilling Callahan has probably left a trail of women in his wake (in the same way that Bluebeard moved from wife to wife after he had disposed of the previous one), but Joanna was charmed by him and gave him a chance. Eventually, however, Joanna came to realise what had caused Bill's previous relationships to fail, his 'secret,' as Bluebeard's wife eventually discovered his terrible secret.