Are bloodmoon's now extremely rare now? I haven't seen one after two play throughs of the game.
blood moons run on a specific set of boolean logic.
when you create a character it adds one to the nerd variable if your character does not have the hidden "cool" variable which is applied to inherently awesome characters.
then when that character enters a world the game checks to see if your character is a nerd, if yes, then you don't get a blood moon.
only nerds don't get blood moons.