The key word is believes and this can be hard to make clear. If you're writing a romance, you know there should be a happily ever after or a happy for now moment. In a mystery, you know the villain will be unmasked. In any genre the time needs to come for one of the characters to realize all is lost. In a science fiction story the hero or heroine might believe the plague will never end. In fantasy, the wizard believes he or she ahs lost their powers. A bit of note for the horror genre. The black moment may become the end of the focus character, What could be blacker than that.
Is it possible to have more than one black moment in a story? Skillfully done, this can be achieved. The character may believe all is lost. Then he or she decides to go on from there. Then, during the new path he or she has taken there comes another time when all can be lost.
Next week, we'll look at how to build a black moment so it's really black.