Display Gutenberg blocks from “Posts Page” above the post list?

You could add something like this before the while()-loop in your index.php:

// only if we are on the posts page
if ( is_home() ) {

    // global post record
    global $post;

    // get the posts-page record
    $post = get_post( get_option( 'page_for_posts' ) );

    // set it up as the global $post
    setup_postdata( $post );

    // content output, cheaply wrapped
    echo '<div class="posts-page-content">';
    the_title();
    the_content();
    echo '</div>';

    // restore the post data to the original one
    wp_reset_postdata();
}

Then you propably have to add some lines of CSS to look it nice and clean.