I recently received a request to add a Blog tab to a space and after lots of fiddling around in the admin console, I somehow managed to do it, but for the life of me, I could not replicate that act on other spaces.
Much searching through the Clearspace documentation finally brought this to light - (http://www.jivesoftware.com/builds/docs/clearspace/latest/ManagingSpaces.html#CreatingaNewSpace). You can only add a Blog tab when you're initially creating a new space, yet somehow I'd done it. How?
After a lot of experimentation, I finally was able to repeat the act.
1. In the admin console, go to Spaces>Settings>Space Settings and choose the subspace where you want the tab to appear.
2. Under "Allowed Content Types", remove the check mark from "Blog Posts".
(We recently migrated to Clearspace (2.5.6) from an old Jive KB product and so our site structure was pretty much set during the migration process. As best I can tell, the migration process set the "Blog Posts" attribute when it created all our sub-spaces - but, for some reason, the Blog tab never appeared in any of the spaces.)
3. Click "Save Changes".
4. Recheck "Blog Posts".
5. Click "Save Changes". The blog tab should now appear on the Clearspace web page.