When you say "blocked", are you meaning to say that the iPad is disabled? IF that is the case, then the content is no longer accessible, so you couldn't back it up now, even if you wanted to. I'm sorry, but the only alternative is to wipe the phone to enable it again. See this support document. If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
You will only be able to restore to your latest backup.