To adjust the blog to include the "Add a Comment" feature on each blog post when displayed through the main blog, change the "Reply Nesting Depth" from the default value of 0 (zero) to 1 (one) in the Blog Options (DiscussionProfile) for your blog.

You can also change the text displayed for this option by change the DiscussionServices configuration in site.xml or your subsite.xml file. The value is configured through the following setting.

<setReplyToReplyTextclass='xsd:string'>Leave a Comment</setReplyToReplyText>