I wanted to do the exact same thing (have the target value on the Add Link page default to _blank) so I Googled and came here. I noticed there was no answer, but it gave me a good starting point and I was able to make it work. First I tried moving the values for _blank from the 1st in the list to the last in the list, like sexinart did, but it didn’t work. Well, here’s my new code that works ONLY AFTER I went in and pretended to edit an existing link. I just clicked the Edit link for it, and re-saved it, and THEN my Add Link interface updated to default to _blank (no matter how many times I refreshed, or logged in and out, it didn’t take my changes until I did that pretend edit).
Here’s the code in edit-link-form.php
<td>
<label>
<input type="radio" name="link_target" value="" <?php echo($link->link_target == '') ?> /> <?php _e('none') ?>
</label>
<label>
<input type="radio" name="link_target" value="_top" <?php echo($link->link_target == '_top') ?> /> <code>_top</code>
</label>
<label>
<input type="radio" name="link_target" value="_blank" checked="checked" <?php echo($link->link_target == '_blank')?> /> <code>_blank</code>
</label>
<?php _e('(Note that the <code>target</code> attribute is illegal in XHTML 1.1 and 1.0 Strict.)') ?>
</td>
Hope this helps someone!