Using a subdomain like urblogname.blogspot.com or foo.subdomain.com is a big disadvantage for any blogger. Its always suggest to use a toplevel Domain Like www.example.com.
If you are using a blogger as your Blogging platform than google allow you to use a custom domain for your blog. You have to buy a domain for your Blog and then you have to make some changes in your blog setting. Soon your blog will live on your custom domain.
Using a top level Domain is very useful for a blogger reputation in Google Indexing. Top level Domain are always preferred over subdomain. As from last year the Google is redirecting its all blogspot.com domain to country specific domain like blogspot.in or blogspot.us so its became very Important for bloggers to use custom domain.
What if you want a custom domain for free.