The problem was a redirect in site-info.php. After that the $_POST variable was empty in the code inserting in the admin_footer.
I just need to use a further hook ‘admin_init’ and save the value here.
add_action('admin_init', 'pg_save_custom_site_options');
function pg_save_custom_site_options(){
global $pagenow;
if( 'site-info.php' == $pagenow &&
isset($_REQUEST['action']) &&
'update-site' == $_REQUEST['action']
) {
// Use a default value here if the field was not submitted.
$new_field_value="0";
if ( isset( $_POST['blog']['blog_order'] ) ) {
$new_field_value = intval( $_POST['blog']['blog_order'] );
// save option into the database
if( is_int($new_field_value) ){
update_blog_option( $_POST['id'], 'blog_order', $new_field_value );
}
}
}
}