blog categories to appear as child under parent blog in navigation

I have my navigation set-up to show my blog. So when you click the link in the navigation it shows you the blog with the 5 most recent posts from the various different categories I created in the blog. What I want to do is have my blog show up in the navigation with a drop down menu of the categories. I have various links in the navigation with drop downs but I cant seem to find out how to create a drop down menu with my blog categories.

Thanks