Some interesting thoughts (by a blogger) about why young students and scholars should avoid blogging:
Academic Blogging: Some Reservations and Lessons
It's good advice for those who do blog too. In its immediacy, blogging can inhibit reflection.
Since I am obviously not following Blum's advice to avoid blogging altogether, I may as well mention that (1) Blum doesn't address the public service aspect of scholarship and (2) while I agree that peer review is important, I am more inclined to put whatever "new" ideas I have out there because I think that there is something fundamentally broken about the way the academic publishing system works (more on this, hopefully, another time). Or maybe it's because I see myself as a גֵּר־וְתוֹשָׁב.
Here is another perspective mentioned in the comments to Blum's post: The Common School.