Black flowers generally refer to the darkest flowers possible, which are not necessarily really black, but often a dark purplish color, instead (which usually appears to be black from a distance). Many people enjoy the contrast that can be created between black and brighter colored flowers, such as yellow and pink. There are several different kinds of "black flowers" to consider.
The bottom section of a black iris appears especially black. However, the black iris is, contrary to its name, rather a deep purple shade. It is part of the Iris genus, and is cultivated in hillsides, by streams, in forests and in meadows. They range in color from reddish purple to dark purple, to a purple that is extremely dark and velvety.
Black tulips are a deep, vibrant purple, upon close inspection, not black at all. However, from a distance, they can appear to be black, and are therefore classified as black flowers. The lighting can also affect how they look. In darker lighting, they look very black, and in bright lighting, they appear to be purple and sometimes even reddish.
Black hollyhocks (also known as Alsea rosea nigra) have extremely dark blossoms, and therefore can appear to be black. The maroon blooms are perhaps some of the darkest of all black flowers. They have big lower leaves that are shaped similarly to hearts, and grow on towering spires or spikes. They are in bloom for a long time, between June and August.
Raven-colored black roses do not actually exist in nature yet, although cultivators are hard at work at them. Black roses are actually a deep, vibrant red as opposed to black. Some examples of "black roses" include "Ruby Celebration" and "Black Magic," both of which are rich reddish shades. Some florists dye these roses jet black, however.