It will block them, the only place/type that doesn't get censored is when wrapped around CODE tags, phpBB3 failed at this feature, for some reason they decided that domains should not be censored.

So if you add in censor list "google.com", it'll be censored most of the time, just not when it's wrapped with CODE tags.

Solutions:
You basically have two solutions, either add the domain without the extension, ie. add google not google.com, or modify the current censor system code to match your needs.