set page permission to make it visible to all users

I assume you already read the intro documentation to using permissions "Using the Lithium Permission System" and "Permission descriptions". In addition to these there's a more practical example documentation "Set forum permissions" that can be easily applied to your blog scenario as well.

Basically what you need:

1. At the blog level where you want to set permissions, go to Community Admin > Users > Permissions > Defaults.
2. Scroll down to the Forums (Board) permission section.
3. Set the permission settings as needed.

If you want only the blog in a category to be visible to anonymous users you start at category level by denying the read permission for the default role. Then head over to editing permissions on blog level and grant the permission there.