Firstly, take a good look at the other colors in your garden before you decide to go with a black accent. It is great if you have other brightly colored flowers in clusters as you really want to achieve wonderful contrast. Also, it is then very important where you choose to put your black accent, and what you decide to use, e.g. flowers, ornamental grasses, shrubs etc.
The article below taken from Balcony Garden Web, gives 20 ideas for incorporating black into your garden.
First thing first, black plants are not really black, but dark purple, deep burgundy, maroon or red. These type of flowers and plants of black color can transform any garden or container garden in an exquisite way, they add a tropical touch and look exceptional when grown with other bright colored plants.
Beautiful and dramatic, this tremendous closest to black flower appears in deep maroon color in spring. This variety can be mixed with white or pink tulips or other bright colored flower to create an astonishing view.
Good thing is that it is a low maintenance plant and usually easy to grow, that makes it a good plant for beginners. This fairly cold hardy plant blooms in mid or late spring under USDA Zones 3 to 8.
To see the full list of black flowers, head over to the Balcony Garden Web.