I think you might just have to delete each one individually. Do this by clicking on your blog, and then clicking on the permalink that's below each one of your blogs. Then click "manage this entry":
A page should pop up that has four options on the left corner of the page ("edit this blog post", "view edit-history", "delete this blog post" and "un-publish this blog post"):
Click on "delete this blog post", and do it for each one. If there is a simpler way, most likely one of the admins will let you know.