I think one of the main reasons I would use a blog in my future classroom is for class-related information. For example class schedules so that the parents know what we're doing, and calendars so that they know when we are doing particular activities. You can also use the calendar for important dates. I could also post the child's homework on the blog just in case they forget to write it down or or leave their study buddy at school.
A more interactive way I could use a blog in the classroom would be for discussion questions. Students have a lot to say and sometimes there isn't enough time to get to every child during class. With a blog the student can fully express their feelings on the the topic at hand. Another way to use a blog in the class would be for daily/weekly journal entries. The students could write about something as simple as how their day/week was. Or it could be a little more complex, and you could have the students make up a story. Just something that would get the students writing and thinking "out of the box". One way that is truly interactive is to have a class blog, and link with other classes. These classes could be down the hall, in a different county, state, or country. The students could get a glimpse of how other students around the world learn, and in turn it benefits them because they may learn something they didn't know.