If you only discovered Safe HTML after already putting a dodgy script into an HTML block and can't get back by reverting the page version, you can use the Safe block template to create an override of the core HTML block view, so making every HTML block on your site use Safe HTML.
Use FTP or your host control panel to copy
This will create an override of the core HTML block view making it 'safe'. You can then edit the afflicted pages and use block design to change problematic HTML block's templates to use the Safe template. Once finished, simply delete /application/blocks/html/view.php and your other HTML blocks will revert to the default.