Show block on specific pages URL attribute
You can use PHP visibility on your block settings to achieve this. Drupal block's visibility does not provide the functionality to display blocks based on parameters after ? (aside from ?q= of course) so you need to do this with custom PHP. 

Set your block visibility to use PHP as the markup and place this in your code:

&lt;?php 
// Show the block if the 'showblock' attribute is available
if (!empty($_GET['showblock'])) {
  return TRUE;
}
return FALSE;

?&gt;


Happy Programming!