Getting a Completely Black Background - Purple Iris
I used only 1 flash to take this photo. I used flash to overpower the ambient light. I placed a softbox 90 degrees to the right side of the flower and a reflector 90 degrees to the left of the flower. I made sure none of the light spills on the background to ensure a complete black background.
If no light reaches the background then it will fall into darkness. With photoshop you can make some adjustments to get the background completely black.
This photo was taken in September 2013 with a Canon 5D MkIII and Canon EF 24-105mm f/4L IS USM @ 105mm
Camera settings: f/22.0, 1/125 and ISO 125