If blogging can be hazardous to your job (as this article says: Blogging may be hazardous to your job By Amy Rosewater, The Baltimore Sun) just how is that so? And, don't we have freedom of speech engraved in the U.S. Constitution?
What the article talks about is bitching about co-workers etc on the blog. That people get fired over that.
A lot of people feel irritated etc over their job. No doubt bitching about your boss or coworkers has happened throughout history. Maybe that's why Judas turned Jesus in to the authorities?
Seriously, what's important is to consider how you handle the inevitable irritation you have over co-workers and your boss. For example, therapy? There's a zillion ways of working through emotional duress that don't involve bitching in public.
When you're writing a blog it may seem you're in private. Maybe you've locked the door to the room, it's late at night, etc and nobody is around and you can pour out your deepest thoughts. But, really, who is your audience? Once that blog posting hits the web, it's public.
If you want to use writing as therapy, get one of those blank journaling books.
Do you really want to post your therapeutic writing for the whole world to see?