Without a screenshot, system info and other details nobody can say. We don't even know what PS version you use.
The black square is possibly a monitor card limitation, or something that can be fixed in Photoshop Preferences. Try Photoshop > Preferences > Performance, and maybe turn off Use Graphics Processor, or alternatively, turn off some of the choices in Advanced Settings, to see if things improve.
Are you using NIK plugins?
Google updated the plugins recently to correct this problem:
Version 1.109 (December 11/2013) is the latest:
Fixed black or white tile artifacts that might have occurred after returning to Photoshop from plug-ins
You can find out the version you have installed by launching Photoshop:
On Mac: Select Photoshop>About Plug-in>Analog Efex Pro… (or any other NIK Plug-in)
On Windows: Select Help>About Plug-in>Analog Efex Pro… (or any other NIK Plug-in)