Cailan says, he isn't sure that this is a Blight. But even after that he says 'The Blight ends here!' So while he's not sure about it he does consider the possibility, and rightfully so.

And after Ostagar there is no nebulous knowledge. A Darkspawn horde, creatures the people believed extinct, just outnumbered the Fereldan armies. If Loghain has any doubts about what is happening all he needs to do is to send scouts and gather information.