Blogger.com offers a good commenting system built into it's blogging platform. You can find the link to "Post a Comment" under every blog post. Clicking on the link will open a new page where you can enter your comments and read what others have commented on that post.
Depending on the security settings of blog, you might have to log into your blogger account and/or pass the visual verification process to post your comment. Additionally, you might have to wait for the comments to be moderated by the blog author, before it is added on the comments page. To discourage spamming in comments, all links on the comments will automatically carry a "rel=nofollow" attribute.
