From the url:
Opening too many windows leaves some of them black
This is a bug in the Nvidia drivers (as of now from 9625 beta too 9631 for legacy and up for others) that may be fixed in the future. This is because your video card doesn't have enough video memory. When you've used it all up with open windows, newly opened windows are left black. Reloading the window manager or resizing windows smaller or minimize other opened windows will make the currently-opened windows visible, but doesn't solve the problem. Some have had success with one of the following, though usually at the expense of a lower framerate:
* beryl --indirect-rendering
* beryl --strict-binding
* beryl --indirect-rendering --strict-binding
* beryl --force-aiglx
* beryl --use-copy
If you use the Blur plugin turning it off is beneficial since it leaves you with more video memory for your windows.
Seems to me, that it is a bug in nvidia-drivers..