$output .= "<h3 class='grid-entry-title entry-title' $markup><a href='{$title_link}' title='".esc_attr(strip_tags($title))."'>".$title."</a></h3>";



change it to


$output .= "<h3 class='grid-entry-title entry-title' $markup><a target='_blank' href='{$title_link}' title='".esc_attr(strip_tags($title))."'>".$title."</a></h3>";