Block missing after upgrade to 4.7

I couldn't figure out why my blocks weren't appearing. Thought it was a bug with XTemplate but then came across this comment:

If when you change to a theme, and your left/right blocks disappear, you will need to update the blocks table, and set the region column = 'left' for every row which does not have a value.

The proper place for the fix is in updates.inc but I figured I'd start the issue here.