The black moment is, in short, the crisis point of your story. This is where your hero and heroine's roller coaster ride comes to a crashing stop, and all of their worst fears come true. Let's go back to John and Marsha. For John, this may be the point at which his football buddies discover he's written that book of poetry. Maybe he believes he's in danger of losing his valued tough-guy status, and thinks his job might be on the line because no one will believe a softy like him can still kick butt on the field. The fans will hate him, his coach will hate him, and the public in general will look down their noses at him. Bad news all around for John.
Marsha has unintentionally let it slip that the poetry book exists, so John is angry with her and goes back to macho mode. This confirms her worst fears, too. The man she's been falling for throughout the story has turned out to be the worst kind of woman-hurting stud she's seen yet. Maybe she even gets her dream promotion, but by now she doesn't want it. There's no satisfaction in the job, because she hurt someone to get it.
Both of these characters have a personal crisis or black moment. In addition to that, in romance, is the romantic black moment - the point where your hero and heroine believe they'll never get their happy ending in each other's arms. And, of course, they're miserable because of it. Each of them spends more and more time thinking about what might have been, and dwelling on the things they liked about each other. In romance, this downslide leads to a decision on either (or both) character's part to jump that final hurdle and reach the happy ending. That's tomorrow's topic.