No, I got what you were saying. I just disagree that Blackwall would know much more than the average person, even if his wife was a Warden. And I also disagree that he would even speak about what he does know in the prescence of someone he met only a short time ago.
If this scenario is proven true, how much would his wife have even told him? I'm sure he knew about the dreams. He certainly knew a little more than the average person. Probably picked up on the general organization of the Wardens, heard a few things. But I just don't think that being the husband of a Warden would give him THAT much more info on the Wardens than the average Thedosian. And nothing all that current if his wife has been dead for some time. And certainly not anything that he would share with someone he met only a short time ago.
Besides, why would Blackwall start giving out specifics about the Wardens, even if he knew them? The more you lie, the more you have to lie. But if you keep things simple, vague and non-specific, there are no lies to cover up with more lies. For instance, if Blackwall mentioned being able to sense Darkspawn, the Inquisitor might get curious about why, when the party encounters Darkspawn, Blackwall doesn't sense them. Now, Blackwall has to lie again to keep the wool over the Inquisitor's eyes. But if he doesn't tell the Inquisitor, then there's no lie to cover up. And the Inquisitor may be blissfully ignorant of the fact that Wardens sense Darkspawn, and be none the wiser.
So, regardless of what Blackwall actually knows, he has a very good reason not to share the info if he's not actually a Warden. And if he is, they're a secretive lot to begin with. Few of them are the sharing type.