How do you know you are blacklisted? Here is a link to check, http://www.blacklistalert.org/

I am not too familiar with Google Apps mailing but if you have access to the mail logs that would be a good way to backtrack what happened.

The logs usually contain server responses to your sends with instructions/link on how to be "safelisted" or at least a reason of why you were blocked. Determine the domains and go from there.