@ash: Considering that he was wanted in Orlais for a really serious crime (the Callier massacre), he (and this is what I get from what he said) seemed to believe that the Wardens in Val Chevin might not have believed him when he said that the real Blackwall had recruited him. They might even have believed that he'd killed the real Blackwall.
While the Wardens do take anyone and everyone, you're talking about one missing Warden-Constable versus the word of a wanted murderer that all of Orlais kind of wanted dead at the time. Sure, he was obviously telling the truth about Blackwall, but I don't think that he was going to gamble on it at the time.
EDIT: lol@the comic btw.
True, but it did not stop Blackwall from recruiting Rainier. And I think taking Blackwall's personality with a risk of running into real Wardens was a more dangerous gamble than growing a beard and knocking at Val Chevin saying: "Guys, are you looking for new recruits?"
Probably this decision was more emotional, than logical. "Oh, no, now this Warden is dead because of me".
Not Blackwall related, but for people who only visit this thread: heads up for Patch 3
http://blog.bioware....-patch-3-notes/