Inside Out - S4-E17
Angel: All this time, it was you, wasn't it?
Cordelia: Took you long enough to figure it out. But nice turn with the Lorne bait. You know, there was a time I would have seen that one coming eons before it ever crossed your tiny little mind.
Angel: Because you're so clever.
Cordelia: On a scale of you to me? Pretty damn.
Answer: Due to his epiphany, Angel basically had the 'true darkness' he worked so hard to reattain ripped away from him, like his soul was after sleeping with Buffy.