There are no natural black roses. Some are so dark red or maroon that they appear black. Much depends on time of year, weather, and temperature. I was just lucky to get bushes that bloomed with really dark flowers. A lot of iron in the soil will darken already dark roses. I use bloodmeal which is rich in natural iron.
You can also purchase black floral spray and color red roses so they appear black.
I did not do that to mine. They grew naturally that way but they are really dark dark red/maroon.