To add a comment to a blog is required send a comment Atom entry document that includes the Id of the blog posting that you are commenting on. Here's one easy way to do it with an asynchronous HTTP (Ajax) request using jQuery.
IBM Connections 4 documentation link: Creating comments
Atom API Request Details
|Content-Type : application/atom+xml|
|Response||Status Code: 201 Created|
Example Atom Entry
Each blog entry has a unique Id, In line 5: <thr:in-reply-to ref="urn:lsid:ibm.com:blogs:entry-4962762c-8a-4bfe563eedba"/> is necessary to include that Id.
Example using jQuery
Note: console.log can throw errors in IE, they are there for illustrative purposes only and should be changed as required.