Ah i see your problem in your screenshot, you have set “Textured” display in the Texture Paint Mode.
Since several 2.6 release, Textured display mode is affected by light sources.
Meaning that if you have no light, the whole cube would appear black, and if you have a light somewhere, only the face exposed to it would be visible, the other faces are under the shadow, and why they are black.
3 solutions :
-In Texture Paint Mode, instead of setting Textured display, keep it to Solid display, the Texture Paint Mode by default enable “Textured Solid”, even if you have not enabled it yourself in the N -> Display.
-add a Material to the cube and set it to Shadeless if you want to keep using the Textured display mode
-If you want your model to be affected by light sources, add additional light sources so the face that are in shadow are affected and show up