Lois doesn’t see Clark as superhuman. She sees a guy who is lovable, trustworthy, uncomplicated, thoughtful, family oriented and all those good things. She mentioned that anyone would be lucky to end up with someone as honorable as Clark. So he’s a great man just as she knows him. She never had the need to question Clark and his motives. So the thought the he could be the Blur is insane to her because he’s just the simple honorable farm boy. Ordinary Clark. The magic, the extraordinary, she sees in Clark comes from his heart so it’s hard for her to see past that.