Minimized FocusPocus and restore the window will cause the Mac desktop to turn black.

In new released system Mac OSX 10.9.3, FocusPocus application has one tiny issue. After launched the application, if you minimize the window and reopen it, the whole desktop will turn black.
In this case, please click "option+Command+Esc" and select "FocusPocus" force quit the application, then your computer will return to normal.