It’s not just the readers who have to think there’s no hope for the hero and heroine to reunite. The characters themselves truly believe they’ve lost each other and involve the reader in the hell they’re going though as a result.
The black moment also has to be believable and connected to the rest of the story. Too simple a ‘crisis’, such as a misunderstanding which could easily be cleared up, can annoy the reader. An unrelated crisis e.g. the heroine is in a car crash and the hero rushes to her side and ‘all is forgiven’ is unsatisfying. And some amazing 'coincidence' would be out of place too.
The real (IMO) ‘black moment’ comes near the end of a novel and needs to be something the hero and heroine have to push their way through or past to earn their HEA ending. In doing so, they learn something about themselves which enables them to resolve their differences.
Wham! How on earth are they ever going to resolve this and get together for their HEA? That, to me, is the black moment.