This bit from Vygotsky supports the benefits many of us hope students will receive from blogging:
Writing should be meaningful for children.. an intrinsic need should be aroused in them, and that writing should be incorporated into a task that is necessary and relevant for life. Only then can we be certain that it will develop not as a matter of hand and finger habits but as a really new and complex form of speech.
A blog can offer both these things for students… blogs can be meaningful to students, especially if they are offered an element of choice (or inquiry) in their writing. Blogs also have a sort of inherent relevance because they are published online where others can discover and read them… but this too can be enhanced if the students are writing with an authentic purpose, even if this is to share their knowledge or thinking.
Vygotsky, L. S. (1978). Mind in society: the development of higher psychological processes. Cambridge, Ma: Harvard University Press.