I remember seeing a video on Youtube about making multicolored roses. Basically they split the stem and put each arm of the split into a cup of food coloring, and the rose would suck up the color. They were doing this with white roses, but I suspect that is what they are doing with the red and black roses in the links above. They're putting a red rose stem in black food coloring and pulling it before the petals become saturated. I prefer the natural color - but I thought the process interesting.
On the other hand, some Gallicas come close to being black.