Pitch Black set up the character of Riddick. Vin Diesel is a big D&D guy, so building iconic characters and worlds is something that he sinks a lot of heart into, and David Twohy's really good at doing that. What happened with Chronicles is essentially that they lost some of the creative options due to having their hands tied by the studio (hence the shift in tone, and moving into a PG-13 rating), although the thematic shift to a larger scale was always a part of what the story set out for - the idea of going to explore the Underverse, etc.
Riddick basically got the two of them back together. Vin Diesel did an open question to Facebook about the possibility of doing them film, and only making anything off of it if it does well in theaters (making nothing at all while doing it). That got a really good fan response - Chronicles becoming a bit of a cult hit on DVD helped.
So, they're getting back to the basics with Riddick, and it's smaller in scope than Chronicles, but not as claustrophobic as Pitch Black was. Hopefully they hit a good stride, and since they've got the rating and creative control they want, I'd like nothing more than to see them be able to delve into Riddick's universe a bit more.
To directly address the questions - yes, Riddick being the leader of the Necromonger army is going to be addressed during the film, so it is a sequel to both Pitch Black and Chronicles of Riddick.
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