I want to block traffic and visitors to my blogger.com blog from a certain country. In this case, those from .ru (Russian domains).
No, you can't. You need access to the server administration tools (like .htaccess ) to flawlessly block IPs from a certain country.
Anyway, blocking an entire country is not something I would suggest you to do, especially if you are only having problems with spam. What about all valid Russian-speaking people with good intentions that would be blocked? If that's the case (spam), I suggest you to instead replace the default Blogger system with the Disqus commenting system. It has many features, including a strong antispam protection that let you set blacklists.