* When you enable a block you usually want it in the "public" (frontend) area or in the "private" (backoffice) area, but rarely in both. You now have a left sidebar zone and a right sidebar zone (and the new ones): I would make use of something related, an admin area (a more suitable name surely may be found).
By default, when you create a block you must choose in what area you want it to show: frontend, back, or both (for the backoffice you would make use of the previously commented list of regexps, and you would not need to copy&paste the same list for every block).
And this idea might lead us to the concept of subtemplates or subthemes, as other CMSs have, but I guess is a completely different topic (although in someway related, as you could consider this whole admin thing as a subtheme...)
With the new regions in core, you can do admin regions (or admin-left, admin-right), which only show up on admin pages. Goba