Yes, your boss can definitely penalize you for blogging about your job. In fact, the company could sue you for libel.
They won't actually do that— it's a waste of their time and money. But certainly they could fire you. Almost all employment contracts have clauses regarding insubordination and public criticism of the company. And most contracts are "hire at will," meaning that the company can terminate your employment at any time for almost any reason.
In short, you were fortunate to keep your job.
Don't even think about "venting" about your job on the Internet again. It's not worth it. It's not just your current employers who will find out! Your potential future employers and coworkers will also be searching Google for your name, and even if you don't name names yourself, others may innocently do so when linking to your blog.
When you need to vent, sit down face-to-face with a close friend who does not work with you, or keep an old-fashioned diary.
Don't use "locked posts" for stuff with real-life consequences unless you are absolutely sure you have mastered the technology and you fully trust the host of the system you're posting on to keep those posts locked. But even LiveJournal— famous for its excellent post-locking system— could somehow screw up and unlock everything one day (oh, I doubt it, but it's always possible). So the best advice is to keep your work-related comments off the Internet. At least until you master the art of locking things correctly and avoiding the telltale clues that led your boss to your blog.
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