Most folks know that it's a good idea to use a certain amount of discretion and not to dis the hand that feeds you, such as your boss or employer, certainly not within hearing. But blogging is perhaps more readily felt by the blogger to be an exercise in free expression, guaranteed under the First Amendment to the U.S. Constitution.
The Constitution may guarantee you the right to bad-mouth your employer, but it does not seem to have a provision protecting your job once you make yourself persona non grata over some inconvenient thing you've revealed.
For a discussion of related issues, see the article below from from CNN-Money, featuring employment lawyers and First Amendment expert Eugene Volokh of UCLA Law and the Volokh Conspiracy blog.