The administrator of a page/a community decides whether or not comments should be permitted for the respective blog app. Only when the administrator has approved this is the comment function displayed below the blog article.
To approve comments, first go to the app settings. Then add the check mark for the option "Comments".
Notifications for comments
Users who have subscribed to (identifiable by the blue bell symbol) or commented on a blog article receive a notification when further comments are added to the post.