Forgive me for asking the obvious, but if I understand correctly, you want to block youtube, but allow other sites with embedded youtube videos to play correctly? Or is it ok to allow all of youtube?
The bypass method Gabriel mentioned above will allow either based on hosts or on destination website, so you can put youtube.com in “sites without proxy”, or you can put the individual user IP in “hosts without proxy”. The first method will allow all users to access youtube, the second method will allow one (or more) users to access all internet without restriction.
Another way is, if you are using the content filter, then you can put youtube.com in the “global whitelist”. Again, this will allow all users to access youtube.
If you want to keep youtube blocked, but only want to allow embedded youtube videos, maybe this link will help you figure it out:
Try adding all the links mentioned there in your “global whitelist” of the content filter.