If you move your flower as far away as is reasonable from your backdrop, the backdrop can then be out of focus and will just look black. If you can go to local AOS judging, introduce yourself to the awards photographer and ask them if you could watch them take photos next time. If the photographer is in a good mood and not too stressed they will probably be willing to discuss some pointers with you. Depends how many awarded plants there are and how much time they have to take the photos.