Now, a black screen could have multiple causes. The video driver may have crashed. Or the video driver could be working fine, but the compositor has crashed, so that nothing is being given to the video driver. Or the compositor could be working fine, but the shell has crashed, so the compositor has nothing to render. Or the shell could be running, but it simply forgot to put something on the screen.
For that last case, the Windows 8 shell created a backstop window that sat at a layer below all of the other layers. If none of the other layers were present, then at least you got a backstop window. And in early debug builds, that backstop window contained an ASCII drawing of cats. That way, if you saw cats, you knew that you were in that last failure case: The shell is running but forgot to put something on the screen.
I approve of cats.