I was playing around with one of my favorite apps Any Cut. Regrettably, Any Cut (which isn't available in the Play Store any more) doesn't work correctly on Jelly Bean, but it works enough to be able to take a look at many of the shortcuts it's able to find. And low and behold, Blocking Mode was in the list. Since Any Cut is problematic on Jelly Bean (it kind of works, but not completely), I found another app on the Play Store that operates in much the same way that Any Cut does - QuickShortcutMaker.
No root required!
Here's how to get to it:
1) Download QuickShortcutMaker from the Play Store. (Note: the app closed on me a few times while trying to use it, but it eventually worked)
2) After QuickShortcutMaker installs, open it up.
3) Type Blocking Mode in the text field. The app is pretty slow when it comes to populating the available shortcuts, so it might take a little bit. Once it finishes, tap on the first result at the top (It will say "Settings" with Blocking mode underneath it.
4) Rename the shortcut to whatever you want. It's named "Settings" by default, and I changed mine to "Blocking Mode."
5) Tap "OK."
6) You'll now have a shortcut on your homescreen that will take you right into Blocking Mode (the icon will be a cog just like the Settings icon). Open it up and and block away! (Note: the shortcut on your homescreen will be the only way to access Blocking Mode, so if you delete it, you'll have to create a new one in order to get back into the settings for it)
The app will force close when trying to add contacts to the custom allowed contact list. But it seems to work fine when selecting "Favorites" as opposed to creating a new custom list. So if adding contacts to a custom list is force closing for you, add the contacts that you want to come through to your general favorites list when Blocking Mode is enabled, and then use the Favorites option in the Blocking Mode settings.