Black dice usually represent outside factors that work against the character – the howling gales, dim light when trying to find a stowaway in the hold, poor handwriting when attempting to decipher a treasure map, etc. Black dice don’t actually have to be black, as long as they’re a different color from the dice you normally roll, but sinister is better! They’ll be referred to as black dice throughout.
If the GM feels there should be a penalty to a roll, he will assign a number of black dice, which are rolled at the same time as the normal dice. Each success on a black die (a 1 or 6) cancels out a success on a normal die. But black dice do not get rerolled on a 6, you’ll be pleased to know.