There are different types of blog but basically, at least in the beginning, in terms of the learners, I see this as a blog where students can get writing practice, have what they have written read by other students and then with the comment features of the blog, more interaction and communication in the target language is promoted.
It is probably a good idea to begin by asking your students to write on a specific activity or topic, providing them with guidelines in the form of some questions they could answer. This will motivate them and help them to focus, at least in the beginning. Expressing opinions via blogs has also been shown to help with language learning because being able to express themselves online increases their level of confidence (see this this link for a useful discussion of how language learning takes place by blogging).
You could give also give them a project to do that would, for example, develop writing and research skills, and the blog would then also be used to showcase their work as well as providing an online resource for use by others. You can post different specific ideas here that would be appropriate for the different levels of students you have and that would also reflect the needs of the curriculum.
Another way you could use the blog with your students would be as an on-line personal diary for students, a place where they could write an account of their learning via the blog. We know from the research that when students monitor and evaluate their learning they learn better, and that they also learn better when they when keep a written record of what they do.
Specific activities you think you could use for language learning via this blog please!