In some sense, a blog contains a series of articles.
I believe you need to be focused on your title whenever you are writing an article. You also need to ask what is the objective of writing the article in the first place. Check out eZineArticles for some tips on how to write a good article =)
If you are given a job to write a blog post, then you will also get the topic on which you need to write. In both simple article and in a blog post, the main purpose is to write useful/fresh/informative content on the given topic. This can then be called as blog post or a article Both are quite the same thing.
I think even though both are the same thing, the two words carry different connotations. Blogs have a reputation of being opinionated or information. The word article is associated with journalism, which is objective and has rigid standards.