How many of the refugees (I'm inclined to think they would flee to the nearest nations like Orlais, Free Marches, Nevarra and stop there) could actually reach the top levels of the governments in the nations? So, yeah, the news of the Blight would remain in rumors, and, like I said, sending an army is costly, would take time, and for no forseeable gain except for the gratitude of Ferelden. Shortsighted, yes, but that's government for you.