You can still use iWeb to create and maintain a blog if you don't want to allow comments by visitors. Otherwise you'll need to use one of the established blogging sites.
You could add a separate comments page with links to it on your blog pages. For that you can use DISQUS - Elevating the discussion or IntenseDebate comments enhance and encourage conversation on your blog or website.
If you use an established blogging site it can be embedded into your iWeb page as shown in this demo page: Embed a Site Within an iWeb Page