Just learned today that by default you can't store files at the root in Windows Azure storage. So, what if you have http://foo.blob.core.windows.net which you've mapped to http://blob.foo.com and you want to put a file so that it resolves to to http://blob.foo.com/bar.png? Well you can create a special container called $root, which will map to the root. If you do that, anything in the $root container will resolve as expected, so that http://blob.foo.com/bar.png would work.
If you are using Cloud Storage Studio, their UI supports this with the New Root Container option when creating containers.