There is a special container name $root that you use to write to the root container. See this MSDN article for more information about this root container.
As long as you can store static web content in blob storage, there is no concept of a default object; that is, you cannot have something like default.html if a visitor goes to www.YourStaticSite.com. They need to visit www.YourStaticSite.com/default.html.
EDIT: As mentioned in @SantiagoIT, you need to place the default page somewhere (like a role on the Internet). Since you can move static content to blob storage (e.g. css, javascript, images and static html), this will reduce the load on your web role instances (since blob storage requests go directly to blob storage, bypassing your web role instances at all). Even if your site is hosted locally, you can use the blob repository for the same benefits.
Going further: when the content is in the blob repository, you can activate the CDN (edge cache). For frequently visited content, this can lead to increased productivity for your end users.