If I remember correctly the description for the challenge said a black object. Other than that I like the contrast between yellow, white, and black, makes the flowers stand out. You could have positioned them better, being in the middle of the frame doesn't help much. The lighting on that bottom flower is a little too dark, some light would help it come out more, unless you wanted it to be dark.
I like this picture. At first I interepreted the challenge in the exact same way, and then decided the object should be black. But after reading all of the forums on this before hand...I would have done a little spot editing here as long as you could have
Actually, the flowers look pink. It seems that the main subject should be reflected in the challenge, not the background. Just my interpretation. Plus you have a small white speck to the right of the flowers. I spent 30 seconds trying to wipe it off my monitor (exageration).
The framing seems a little off kilter to me, perhaps a square frame a bit tigher around the flowers, centered perfectly would be nice. It seems a little oversharpened, too... there's some aliasing along the edges of the flower petals. I like the subtle shades of color in the petals, too... though the picture on the whole doesn't really say 'black' to me. :)
There seem to be a lot of blue marks all over the black part of the image. And it's really distracting, as well as some whitish/brownish marks on the bottom.