Drupal Block on Specific CiviEvent Page (How to?)
One option is to use .extra.tpl files to load the data you wish to display, rather than the block.  An explanation of the technique is here.  This avoids having to worry about whether something that's in scope for CiviCRM isn't in scope for the PHP visibility block.

Another is to check the value of the $_REQUEST or $_POST supervariables for some element that is specific to this page (e.g. I'm guessing the page ID is in there somewhere).  If so, you could test for the presence of that value.