So, you'll need two groups for that.
The "registered users group" and (lets call it) the "Authors" group.
Then you set up forum permissions for the "Blog" forum.
This can be done under: ACP > Permissions > Forum permissions > "Blog" forum.
Then select the registered users group and the Authors group, so we can set them both at once.
Then under the "advanced permissions" you'll find various tabs, of which one is the Post tab.
For the Registered users group you can "Start new topics" to "No" and "Can reply to topics" to yes.
For the Author group you can set the "Start new topics" to "Yes".
And that's it! Add your authors to the Authors group and they can start topics.
While the 'plebs' can only reply!