OK seem to have found it.
If you have access to the server / code, then as a temporary "hack fix" until it gets resolved properly just move the blocked.gif to a publicly accessible location and just hardcode it in...
program\steps\mail\func.inc
// clean HTML with washhtml by Frederic Motte
$wash_opts = array(
'show_washed' => false,
'allow_remote' => $p['safe'],
'blocked_src' => "./program/blocked.gif",
'charset' => RCMAIL_CHARSET,
'cid_map' => $cid_replaces,
'html_elements' => array('body'),
);