Doesn't affect block content

The solution is simple, I'll add the code inline as well as in a patch...

Find this code in filter_default.module:

<?php
function _filter_default_is_new($form_id, $form) {
  switch ($form['#id']) {
    case 'comment-form':
      return empty($form['cid']['#value']);
    case 'node-form':
      return empty($form['nid']['#value']);

  }
  return FALSE;
}
?>

And add the following two lines alongside the existing 'case' statements:

<?php
    case 'block-box-form':
      return empty($form['bid']['#value']);
?>

It's up to the maintainer, bjaspan, to update the module.