Nowadays, blogs are one of important web services to publish and share various information. Accordingly, evaluation of various keywords in blogs is one of the important research topics for effective and efficient classification and retrieval of blogs in the blogosphere. In this paper, we propose a method to identify important keywords in a blog. In order to identify such keywords, we consider web context, assuming that the blogs documents are generated from web contexts by proposed generative model. Therefore, if the contexts of keyword on the web are reflected well in the blog, then we may regard the keyword is essential because the blog is rich on the keyword. We clustered the blog articles on the given keyword by several subtopics using LDA (Latent Dirichlet Analysis), and compared the clusters with the web context documents obtained by web search. Finally, we evaluated the richness of blog on each keyword.