Yesterday .blog namespace (brought to us by Automattic – company behind WordPress) was open for public registration. .blog is perfect top level domain to register domain for your blog – especially if .com is already taken. So go and grab your .blog domain while it is available!
Prices vary by registrar. The cheapest solution I found (spent few hours reviewing) was publicdomainregistry.com – $20 per domain. If you find cheaper option, let us all know in comments!
UPD: it seems that publicdomainregistry.com offers only domains for bulk buyers and re-sellers, so next best option is zuuq.com – $22 for .blog domain.