I found this rubric on Education World for grading blog posts. I thought it was an interesting way to hold students accountable for what they are posting. Grading or at least having standards for each blog post would be my concern. You may have students that write a lot and then students who do not write enough. Having a rubric would help all students identify your expectations as a teacher as well as making sure their comments are appropriate. Obviously this rubric will need to be changed to fit your specific assignment, but is a great jumping off point.
Here is another example of a blogging rubric.