The Blog Tile is a link to the home page of your blog and it appears on every page in your blog. That means the Blog Title just above the header image is the "home" link. No matter which page you are on if you click the blog title you will return to the front page of the blog.
You can add another Page tab to your top navigation menu to a contact page. See here for creating a page > http://en.support.wordpress.com/pages/
See here for a contact form. http://en.support.wordpress.com/contact-form/
The easy approach is to set up a custom menu.
1. You create a custom menu. http://en.support.wordpress.com/menus/
2. You add the Parent pages you want to display and arrange them in the order you want.
3. Then you add and drop and drag the sub-pages below the appropriate Parent pages, and when you have everything arranged in the menu that you want you save the menu.
4. Go to the “theme location” module at upper left on the menu page. Select your custom menu name from the pull-down labeled “primary location.” Click the save button in that module.
5. Refresh your browser so that it isn’t possibly pulling a cached page, and view see your new menu with Parent pages and drop-downs to sub-pages.