To make a site collection able to be viewed by anyone you have to enable Anonymous acces. See this link for a guide from Randy Drisgill.
Basically you do the following steps:
- From the web application settings in CA, you edit the authentication provider for the web application hosting you blog. In this settings you enable anonymous acces.
- Next you go to the site settings -> Site permissions of the blog, here you can click the "Anonymous acces" button on the ribbon, from there set anonymous acces for the entire web site.