Remember from HBP, where Snape tells Draco that it seems his mother's been teaching him oclumency, since he couldn't break into Draco's mind?
Mark
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.