There is no expiration date on blogs unused or otherwise. If you can make contact on your own with the owner of an unused blog then there is a means of a consenting blog owner to transfer the blog to another username account. http://en.support.wordpress.com/moving-a-blog/#transferring-your-blog-to-another-user-or-account In the case that there is no contact information Staff will not breach the privacy policy and provide you with an email address.

if you are not successful, you can register another blog with a similar name. http://en.support.wordpress.com/register-a-blog/

Or you can register a domain via a domain mapping upgrade and it will not matter what the underlying .wordpress.com address is.
http://en.support.wordpress.com/domain-mapping/
http://en.support.wordpress.com/domain-mapping/register-domain/

Can you change it to my choice?
I hope you can - I would like to stay with you :)
Blog url: http://mattiadotcom1.wordpress.com/