It seems to me that there are two different issues at play here, the noun “a blog” and the verb “to blog.” The first is relatively easy, while the second is more complex, more difficult to teach and to learn.
For most of their school lives, students are trained to know learning as a noun. They go to a class, write a paper, take a test, listen to a lecture, complete a homework assignment in a closed loop of teacher -> task -> grade. There are occasional excursions outside this loop, for instance when students work together in small groups to critique each other’s papers (not always a useful exercise if they don’t have the tools to recognize what they see and provide useful explanations), or to complete a joint project. But the loop is still there, with the teacher the final word on the value of whatever it is.
A blog is different, but still a noun. However, it is in a different realm altogether: blog posts are public voices. Students need to have the time first to recognize the sounds of their voices, and then to realize that they have become part of a community when they post at all, when someone responds to something posted, or they decide to write a comment to a post. That is the perfect opportunity to start the transition to “blog” as a verb: to remind students that their engaging with others via a blog is a variation of what they are already doing outside of school via text messaging and other interactive communication.
Writing as a tool can’t really be regarded separately: it is the essence of both “a blog” as a public voice and “to blog” is an act of communication — with all the possibilities to come for enrichment of the blog post itself, of the thinking that precedes writing, of the reading that helps prompt writing, of the networking via links and threaded conversations with others.
I agree with Tony: students make some transitions from blogs to blogging naturally and indirectly. More complex uses of blogs that change the focus to the verb — from the writing to the person doing the writing — evolve with time and experience: learning to use links, learning how to read and think and be prompted by ideas rather than assignments, learning to reach out and become part of a network of thinking writers. And all of that must still be in the context of what children (elementary, middle, high school) are capable of doing. Not all learning is readily discernible . . .