Delta Air Lines, Google and other major companies are firing and disciplining employees for what they say about work on their blogs, which are personal sites that often contain a mix of frank commentary, freewheeling opinions and journaling. And it’s hardly just an issue for employees: Some major employers such as IBM are now passing first-of-their-kind employee blogging guidelines designed to prevent problems, such as the online publishing of trade secrets, without stifling the kinds of blogs that can also create valuable buzz about a company.