Mara's blogging plugin does not have any inbuilt support for visitor comments, at least not as yet. It is intended that comments be provided by the Disqus plugin. This can be configured to automatically add a comments section to all blog articles, or to add comments only to specific articles.
In order to have Disqus comments on all articles, add a call to the Disqus plugin to the blog theme. This is the simpler approach as it requires no special code on individual pages. It will suit most sites, with the possible exception of those with sensitive subjects on a few pages.
In order to have comments only on specific pages, add the call to the article file itself. This is perhaps best done by creating a template with the necessary call aready present, and then using this template for new blog articles.
Moderation of comments is performed using the tools on the Disqus website.
Disqus functionality requires a signup at disqus.com and correct configuration of your Mara cms site. See the help page on Disqus setup for more information.