To that, I have to say this: when you become employed, there is a certain degree of right-waiving that you agree to, whether verbal, signatory, or just plain understood. One of those rights is the right to disclose company trade secrets. Another is the right to libel. When you agree to a period of employment with a company, you also agree to waive these rights as they pertain to your employer. If you don’t like this, you can always quit and find another job – you are not guaranteed the right to work for your employer; it is a privilege, and if you screw it up, then they do reserve the right to fire you.
Companies don’t need a blogging policy. If you sign an NDA, then you know you’re not supposed to talk about certain things in your company. If you don’t sign an NDA, then these things are (usually) left as “understood.” Simply agreeing to work for said company means you agree not to reveal their secrets. If you spread bad press about your company on your blog, then you do so at your own risk.
In other words: you should know better! The company shouldn’t have to tell you you could get fired for giving out trade secrets on your blog. They’re going to fire you for doing that whether you blog it or tell your friends. The only difference is that they’re more likely to read it on your blog than to hear it from your friends.
So, just be mindful what you say about your company, and if possible, don’t mention them at all in your blog–unless they pay you to blog about them as a PR tactic.