I'll get right to the point.
drupal_goto(target); Can not accomodate a target="_blank" designation. You can't do it on the server side.

Here's what you can do:

Make some jQuery/javascript that will sniff out your applicable a elements, then force add the target="_blank" attribute.
Here's what I used.

jQuery(window).load(function() {
$('a[href]').each(function() {
var $this = $(this);
var $literatureSwitch = $this.attr("href");
if($literatureSwitch.indexOf('content/literature') !== -1){
$literatureSwitch = $this.attr("href");
// Added a second conditional so that both the content/literature link AND the literal off-site.domain.com link, if entered by a user in an href, will get a _blank attribute.
if($literatureSwitch.indexOf('off-site.domain.com') !== -1){