I'm guessing you want this from the navbar? The just add the About as a menu and blog as a menu item. The blog menu item links to a microflow that opens a newly created form with a url redirector (downloaded from the app store) make sure to fill the url redirector with the desired url and this will automagically take you to the blog site. More details can be found in the documentation for the widget in the appstore.
You can not put a url in the menu. As alternative you can show a form with a url redirector.