You can make blogs hidden by checking the box that reads "Draft (Hidden from Public)" and saving. The url for that blog can be found by clicking the blog's page icon in My Blogs. So they are viewable if you have the link, but they don't appear on your public bloglist. Anyone can make a blog game as long as you avoid blog spam via above. Just a note, blog games are not eligible for the Game-for-a-Game competition.
I don't have a lot of respect for blog games. Especially when they're submitted in game contests which are only for Java Script games.