The blocking criteria is provided by a database your company subscribes to. They can usually choose general categories to block, forums usually isn't one of them because the IT guys usually use them a lot to solve equipment issues. Every so often a site gets put in the wrong category and a customer will complain. The maker will then research it and unblock it if it doesn't apply. That usually doesn't happen in hours though. If it happens in a few hours, it could be someone in IT is screwing with you. Watching you squirm.
If they are blocking forums, then they should already be blocking proxy sites.