Very grey area. Personally, I think if the ump called him out due to blocking the plate. It was the ump that was in a horrible position to make the call. If you look at where the ump is standing, or squatting, how the heck could you call a play from there? If you draw a line continuing the third base line through home plate, he is still on the wrong side of that line. I don't know if this is on purpose to see if there is blocking the plate or not.
So this is a cop out by the ump. He couldn't see squat, called blocking the plate. And because he called blocking the plate, the review has to say yes, because technically the catcher was blocking the plate. So there was nothing to overturn.