I haven;t tested this, but you could edit the "Add Blogs to Search Page" plugin, search for
Code:
if ($datecut == "") {
and replace it with
Code:
if ($vbulletin->userinfo['lastvisit'] != 0) {
$datecut = $vbulletin->userinfo['lastvisit'];
}
else {
This should then only show the new blogs/comments since the last visit - much the same way as a "New Posts" search does...
Let me know how it goes...