When I first think of blogging as a teacher, I think of a teacher to class blog. I'd set up a class blog where I could post information about my class for the students and their parents. There could be a section for assignments in case a student was absent, and a place for me to post important announcements. The main part of the blog could be a place to showcase students' work and what we're doing in class. The blog would be a way for me to communicate with parents, and also a way to positively reinforce the students when they work hard on projects.
I could also see making this blog open to students to post on. It could be a place to have a discussion about a question I post or a place for students to ask and answer questions about something we talked about in class that day. Another great addition could be students' "book talks." We've been learning in my Language Arts methods class about letting students choose their own books to encourage reluctant readers to find books they enjoy and through that learn to love reading. One important aspect of this is "book talks," or a sales pitch for different books that lets students know the wide variety of books they have to chose from. Then students can create a "someday" list of books that they want to read so that they never have to wander around the library wondering what book to choose. I think a blog would be a great way to let students be the ones who shared their own book talks. They would have a chance to write a post trying to convince their classmates to read their favorite book. I think this would be much more interactive and useful than a traditional book report.
A third option would be to have students create their own blogs. Each student could be expected to write so many blogs a week or a month, and also to comment so many times on their classmates' blogs. The blog might not replace a traditional writer's journal, but it could supplement it and help increase student motivation.