I draped a black cloth over a chair and had the plant in front of it. Then used "enough" flash to light the plant and keep the cloth dark. Using a smaller aperture helps for this. Even with that I had to lower my blacks in post processing.
In reality, it would be better to have the black background further away from the plant as it would not receive so much light from the flash.
Beautiful. Both the flower and the rendering. My wife is also quite good at getting orchids to bloom over and over. No mean feat in this climate. The necessary heating means a significant drop of humidity.