When doing a curves adjustment you'll see three eye dropper tools, one with a black mark, one with a white mark and one in between. You use these to set your black, white or midtones.
eg. The background to the image above was grey, so by selecting the black eyedropper and then clicking on the background I was telling it that the tone in the background should now be black.
You can either apply it to the whole image, or to a selection or layer. In my case I did it to the whole image as per the dslrusers.net comp rules, and you can see the side of the cats head has blended into the background.
Alternatively, do the same for white
I hope this is helpful.
Regards,
Owen.
