Michael would never blame himself for the woe he brought on himself (and others, like Kay). But, I doubt he blamed Fredo for the breakup of his marriage--he blamed Kay, using the abortion as an excuse. I used to think he suspected Fredo before Fredo made his fatal gaffe at the Superman show because of what seemed to be hard looks Michael gave Fredo twice in Havana (when Fredo asked who was in Havana, and when he "introduced" Fredo to Ola at the nightclub). But, looking again and again at the scene where he and Fredo go out for a drink changed my mind.