Black flowers do not exist in nature, because flowers need to attract pollinators to survive, and so far no pollinators have co-evolved with black flowers. We have red, yellow, blue and purple flowers because different pollinators are attracted to those colours.
There are many near-black varieties of flowers, the closest to a true black being a few varieties of pansies. If you want black flowers, you can inject a dye that will be taken up into the petals, or a few experiments in soil chemistry (such as adding iron) may give you a darker colour.
Don't know of black roses, but there are some black flowers. Look in google images-- there are lots of black flowers. Some of them are fantasy or art,but some are real. (there's an Irish pub called the Black Rose, but then there's also one called the Silent Woman, so it cold be a fantasy.)
Black roses (roses of black color) do not exist in nature as such, but nevertheless have been created in laboratories. Botanists have manipulated roses' genes through preliminary cross breeding, creating a hybrid black rose. The flowers commonly called black roses are actually a very dark red color. Some roses are called black, but are actually just a dark shade of red, purple, or maroon. To deepen a color of a rose place a dark rose in a vase of water mixed with black ink. Source: http://en.wikipedia.org/wiki/Black_rose_(symbolism)