For the standard sidebar items you can edit the forum_list template.
For any blocks added via add-ons, it depends how they have been added but it is likely that the template modification find and replace code would have to be changed.
Well, at least i know now why it's almost impossible.
So it's my wish for future releases to organize this via Admin-CP just like the order numbers of the nodes...
That would be great and i guess very welcome to any XF admin.