Having just seen the movie yesterday (and my wife asking the same question the moment the credits rolled). I figure I'd chime in.
First, I haven't seen Pitch Black, so I'll confine my comments to The Chronicles of Riddick.
I'd call him CN with both good & evil tendincies. He's VERY independant the entire film, caring little for any of the laws of any of the people he comes across. Personal freedom & choice is his MO the entire film. He's Chaotic, to say the least.
He also kills just for the fun of it, seems to take delight in the suffering of other (particullarly in the beginning of the film). Yet, by the end, he displays an inordinate amount of concern, just not for those he knew (kira, the priest), but for those he barely knows. Ie. the preists kid & wife, the elemental, even some of the prisoners he tried to escape Cremetoria with.
At the start of the film, I'd say he was CN with evil tendencies. By the end, I'd go with CN with good tendencies. Perhaps use Often Chaotic Neutral.
Riddick does acts of both good & evil. He is undecided, his alignment is undergoing transiton between good & evil, yet always seems to drift back to Nuetrality. If you are going to try to confine Riddick within the defintions of D&D's alignment system, your best bet is CN with Evil/Good tendicies.
In other words, don't pidgeon hole the man. (By the way, I was surprised by the movie, much better than I thought it'd be). The necrons would make a cool LE empire to threaten an entire campaign world.
Vraille Darkfang