How did the vision change him? (His unfulfilled vision about saving and curing his people)

Black Elk comes to believe because of ecstatic religious experience that the purpose for his ideal visions is to manifest them in real life through his own work. This leads to a terrible tragedy where he is martyred by fate (and the US government), but he still blames himself, wrongly believing that it was his fault that the Native American culture does not survive. More technically, it's actually more of a United States government issue, not a personal reflection of his merit.