Most studies on tags focus on collaborative tagging systems where each resource (e.g., article, photo) can be tagged by multiple users with multiple tags. The tag usage patterns in self-tagging systems where a resource (e.g., a blog post) can only be tagged by its owner, however, have not been well studied. From the tags assigned by bloggers to their own blog posts, we obtain interesting insights on tag distribution stability and tag clarity. We further discuss the meaning of tag co-occurrences in the context of blogs and argue that tag networks based on co-occurrence from self-tagging system needs to be interpreted differently than that in collaborative tagging systems.