Best way to change the blog page post type?

You may edit the query using the pre_get_posts action. You can do something like this:

add_action('pre_get_posts','wpse_218948_search_filter');

function wpse_218948_search_filter(){
   if( is_page('blog') /* or whatever page it is */ && $query->is_main_query() ){
      $query->set('post_type', 'blog'); # query post type "blog"
      $query->set( 'posts_per_page', -1 ); # display all posts
   }
}