I use a small external flash unit that is held about two feet away from the flower, positioned so that it does not illuminate the ground or anything else other than the flower (I purposefully choose a flower that is not too close to other objects so that I can illuminate only the flower with the flash). Then I use a very fast shutter speed, a small aperture, and a neutral density filter so that all natural light is blocked out and everything that is not illuminated by the flash appears black. So nothing illuminated with natural light appears in the photo but only the things that I have the flash on - everything else appears black.
David