The blog controller is using ChildrenQueryType (https://github.com/ezsystems/ezplatform-com/blob/master/src/AppBundle/QueryType/ChildrenQueryType.php) to list the sub items.
The sort clause is priority (https://github.com/ezsystems/ezplatform-com/blob/master/src/AppBundle/QueryType/ChildrenQueryType.php#L27), it should be "publication_date" of blog post Content Type