For IIS7 what is the default behavior when output caching has no rules added for .aspx files

I think you are misunderstanding pass through authentication. If pass through authentication is enabled, one of 2 things will happen:

• for anonymous authentication, the user ID specified for anonymous access needs access to the physical path for the files. (By default, this is the built in IUSR account, but can be configured in the Edit action for anonymous auth)

• if you have authentication enabled, then the authenticated user credentials need access to the physical path of the files. The application pool identity will also need read-only access to the files.

The <caching> tag that you have enabled in web.config under system.web is .net specific and IIS 7 will not address that caching content. Now, the <caching> you has been configured in system.webServer is handled by IIS 7 module and should work.

If you are looking to enable caching for a specific page like RetrieveBlob.aspx, you will need to add it under <location> tag e.g.:

<location path="RetrieveBlob.aspx">
    <system.webServer>
        <caching>
            <profiles>
                <add extension=".aspx" policy="CacheUntilChange" kernelCachePolicy="CacheUntilChange" duration="00:00:30" varyByQueryString="assetId, assetFileId" />
            </profiles>
        </caching>
    </system.webServer>
</location>

The easiest way to get this done using the IIS 7 UI. Here is my blog which talks about File level authentication in IIS 7, but you can do the same for Output Caching.

Go the Web Site (Default Web Site in our case)
Click Content View
Right click on the file e.g. RetrieveBlob.aspx -> Switch to Features View
Double click Output Caching
Now, click Add... and make the necessary caching changes