Any user can create a Personal Blog and post to it without any special permissions. We recognise moderation of such content is important, which is why you will find a Social Moderators role under the System Tools section of the Manage Security page. Users with this permission can delete Personal Blog Posts but not the Blog itself. Users whose Posts have been deleted in this way will be notified their content has been removed by the Site Moderator.
Community Blog is a multi-area application which means you can customise permissions on an area by area basis. To create a Community Blog, you must have the 'Area Managers' permission in the area you wish to create the new Blog. Once created, the normal content permissions for the area are used to control who can create / delete Blog Posts in that area. If you want users outside of the area to contribute to the Blog, then you can add them as members which grants them contribution rights to that Blog only.