Please read the updates below the post.
For a long time, Google Calendar 'Delegation' (shared) calendars didn't work well with Blotter. The problem stemmed from a long-standing bug in iCal's API (the way Blotter talks with iCal). Fortunately it appears as Apple have addressed this Mountain Lion (Mac OS X 10.8).
If you upgraded from an earlier version of Mac OS X, we suggest that you start by deleting your Google Calendar account from iCal and then follow these instructions for properly configuring iCal.

Once properly configured, got to Calendar -> Preferences -> Accounts (in iCal).

Make sure it says 'CalDAV' below your account to the left. If that is the case, navigate to the 'Delegation' tab (which can be seen above).
Now all you need to do is to select the calendars you want to expose to Blotter. Upon doing so, they should show up in Blotter. Depending on your Blotter settings, you may need to go to Blotter -> Preferences -> Calendars to enable the newly added calendars.
Update: While still unconfirmed, it appears as that some users who upgraded to Mountain Lion from an earlier version of OS X still don't get delegate calendars, while people who did a fresh install of Mountain Lion do.
Update 2: Reader Mike Anderson reached out to us and pointed out to us that he was able to get delegate calendars working on his upgraded Mountain Lion installation after deleting both his iCloud and Google account from iCal and then adding them again. Just removing and re-adding the Google account wasn't sufficient for Mike. Thanks for the information, Mike!
Update 3: We've now received comments from a fair number of users claiming that they are still having issues with delegate calendars. Hence, it appears as Apple didn't fully resolve this issue in Mountain Lion (nor 10.8.1). At this point there isn't much we can do with Blotter 2.1, but we are working hard on trying to find a workaround for this for Blotter 2.2.
Update 4: While inconvenient, one possible workaround for this issue can be found here.