Most camera systems have a contrast based metering system, which is fooled if you have alot of black in your picture. It tries to make that black a nutural gray. This is what is happening here.
To fix this problem use a light meter (if you have one) and meter off the flowers, and not the background, and set your camera to manual mode and shoot that way. If you don't have a light meter, set your EV to - 1 2/3 stops (that is one and two thirds stops and not steps), and check your histogram and adjust acordingly (may be off by 1/3 of a stop).