I am looking to get a webpage to show a custom query while using snippet published here. This is during my functions.php:

function alter_the_query($request) {
    $dummy_query = new WP_Query();
    $dummy_query->parse_query($request);
    // this is the actual manipulation
    if($dummy_query->is_page('blog')) $request['category_name'] = 'Blog';
    // and then
    return $request;
}
add_filter('request','alter_the_query');

The page known as "Blog" includes a fundamental loop inside- so it ought to be exhibiting whatever WP_Query informs it to. However, it is simply exhibiting the Page content- that ought to happen to be overlooked.

I can not appear to locate a reference that describes the qualities from the $request object, so I am unsure things i must do to create this snippet work. Any ideas?

I guess this occurs since you did not change $request['pagename'] value, so wordpress attempt to load content with this pages.

Update this will meet your needs

function alter_the_query($request) {
    $dummy_query = new WP_Query();
    $dummy_query->parse_query($request);
    // this is the actual manipulation
    if($dummy_query->is_page('blog')) {
      $request['category_name'] = 'Blog';
      unset($request['pagename']);
    }
    // and then
    return $request;
}