Actually there are some black flowers in nature but they are rare. Reasons why there may not be many black flowers in nature are that the purpose of flowers is to attract animal pollinators, typically insects or birds. Black seems like it would not be a very effective color to attract pollinators (bright colors are best).

Black flowers are very rare and therefore people have long been fascinated with them. There are also some examples in literature: Alexander Dumas, author of 'The Three Musketeers' and 'The Man in the Iron Mask', also wrote a novel called 'The Black Tulip', about a man who finds a black tulip which is worth a fortune. There are plant breeders that have bred some flowers that are very deep purple or near black. Some examples are a 'Black Parrot' tulip , some near black roses and one company has reported it has genetically engineered a black carnation.