Launchpad uses a blurred version of your Desktop image as its background. Using cmd + option + ctrl + B while in Launchpad cycles through the original image, the blurred one, a black and white version and a blurred black and white version.
With the following defaults write command you can adjust the blur radius:
defaults write com.apple.dock springboard-blur-radius -int XYZ
Replace XYZ with a number between 0 and 255 (0 is the minimum and 255 is the maximum blur).
Restart the Dock to make changes affect:
killall Dock
Restore blur radius with:
defaults delete com.apple.dock springboard-blur-radius
Example:
