CB Blogs is a simple blogging plugin. It does not support custom fields. For that I suggest setting CB Blogs mode to Joomla instead of internal storage. Next you'll need to adjust the content component permissions in Joomla so users have access to the Create Article page. Now you can use Joomla's built in create article page to deal with creating blog posts, which will support all of Joomla's article fields.

You need to use Joomla's create article page not CB Blogs. You'll probably need to adjust the Create Access in CB Blogs so they can't use the CB Blogs create blog page. You'll need a Joomla menu item to Joomla's create article page.