Out of these, Blackout is the darkest, followed by Rated R, then Fame Monster, then Prism (

) at the bottom.
Blackout not only has a dark production, but considering when it was released there's a lot of darkness in its lyricism as well. Gimme More, for example, whilst being a certified BOP, is incredibly dark in the way it deals with the issue of the media + paparazzi. Britney's life was crumbling away at this point, yet she released a song where she encourages the media to "gimme more" is rather dark. Along with songs like Get Naked (I Got a Plan), Get Back, Piece of Me and Freakshow, the album itself is very dark in comparison to her previous releases and those by other girls as well.