Actually, it was Bellatrix who taught Draco Occlumency. I don’t think any talent of Narcissa’a really mattered at that point, though—as a lot of freepers have since pointed out, Voldemort heard exactly what he wanted to hear.
Plus she never turned to look at him when she said that Harry was dead, she was looking toward the castle. He was too shaken to notice, like you said he heard what he wanted to.