This issue goes back to what you are using the blog for. Asking students to write a blog about their learning is a wonderful personal reflective activity for the student. They may be more open in that reflection if they aren't worried about being critiqued by other students.
The teachers believe that interaction with class mates is useful may not be motivation enough to engage the learner. Because our schools are focused on proving academic achievement through marks, our students will rarely engage in online commenting unless they receive marks and some guidelines about what is expected.
Online forums have a different dynamic because they are more like classroom discussions that are less focussed on an individual and more on the back and forth interaction of discussion.