They symbolise power, mystery, elegance, farewell and goodbye. Often the romantic message of black flowers is a negative one, so these should probably be saved for break-ups. However, a single black flower along with a gift of some sort could be a powerful romantic gesture with a hint of mystery.
A pure black flower is the Holy Grail to all flower breeders. To achieve one would be something truly special.
In Victorian and Edwardian times people used to collect black flowers and would go to many lengths to find the most exotic species.