Actually, if I buy a domain like blabla.com, automatically if I search for blabla.wordpress.com will be redirected to blabla.com or I need to pay something? I didn't get it well.
When you create your WordPress.com blog, we create a site address for you, such as example.wordpress.com:
You can also register a custom domain, such as yourgroovydomain.com. Then, when visitors arrive, they’ll see your custom domain in their browser’s address bar instead of the original example.wordpress.com site address:
Note that example.wordpress.com will still work, but all of your traffic will be redirected to yourgroovydomain.com.
I just need to buy a domain, right?