It's not as easy to find a comments rubric on the Internets that can be taught and used in conjunction with a blogging rubric. Langwitches has a commenting rubric that is thorough when assessing content, but like with other blog rubrics, misses the collaboration aspect of blogs and comments.
Here's a comments rubric that my 4th grade students use to assess each other. The focus of this rubric is collaboration to be used in conjunction with blog posts.
Our 4th Grade Comments Rubric: