I am not so sure that the key to student engagement is the "blog," however, I would argue that the key is technology. Technology enables students and teachers to engage in various ways, and can create an organized structure to the instruction.
Face it, technology is not boring, rather technology makes things exciting! Technology also makes things faster, and if handled correctly, makes less busy work for students and teachers alike.
I have no doubt that most classes would succeed and excel with the integration of technology, and blogs are enabled by technology.