Atlus' announcement about its sleeper hit can be interpreted to mean that all players will have their world tendency shifted to black for the day (or a little longer), which affects enemy strength and player stats. A pure black tendency can mean that new passages and enemies may also exist.
"World Tendency" in Demon's Souls is another aspect of the challenging RPG that has a significant impact on the player's experience. If you're a new player who happens to pick up the game on Halloween ... don't panic. Yes, the game is hard, but it'll be a little bit more so with the entire world black aligned.