The black border come from this css:
#cboxContent{margin-top:32px; overflow:visible; background:#000;}
.cboxIframe{background:#fff;}
#cboxError{padding:50px; border:1px solid #ccc;}
#cboxLoadedContent{background:#000; padding:1px;}
#cboxLoadingGraphic{background:url(images/loading.gif) no-repeat center center;}
#cboxLoadingOverlay{background:#000;}
Try this instead;
#cboxContent{margin-top:32px; overflow:visible; }
.cboxIframe{background:#fff;}
#cboxError{padding:50px; border:1px solid #ccc;}
#cboxLoadedContent{ padding:1px;}
#cboxLoadingGraphic{background:url(images/loading.gif) no-repeat center center;}
Fiddle: demo