Comment 30 for bug 1306550

I had the same problem for a while and I've recently found out, that it came from disabling the Unity/Switcher setting to show live previews of the window. This is necessary for (at least some) nVidia users to keep minimizing windows from getting black. However, enabling the Workarounds plugin and activating "Keep previews for minimized windows" solves this problem entirely.
Hope this helps.