I can create a Link on the top level of the menu ok, to /blog/category/main.
What I can not do is add category-Link objects under the main menu item,
because the Link loses its url attribute and points to /blog/main instead.
This is sort of annoying; I might be able to hack it with UPDATE TABLE in SQL
but that's not very user friendly or correct - it would be more valuable to
know why this is what it is and then make it sane.