You can add extra blocks to the category area but they disappear as soon as you turn editing off. This problem has come in 2.1 and I think it's the result of a change in the "Restrict to these page types" to "Display on page types" in the block setting.
When not editing a page it is page type: "course-category" and when in editing mode page type: "admin-course-category" A block can now only be displayed in to location "Any site administration page" or "The current site administration page". In 2.0 you could select "* (Any page)" so it worked.
- Go to a category page
- Turn editing on
- Add a block (e.g. HTML)
- Save the new block - you will see it on the left side
- Turn editing off
The block will be missing.