Indie game storeFree gamesFun gamesHorror games
Game developmentAssetsComics

you only see each face of the cube from one side so that's the direction of the normal, in your case you can see the back of each face too but without a normal pointing back light only reflects off of the outside.

I uninstalled SketchUp so I can't check your SketchUp file but I could probably build a working obj or fbx in Blender if you don't understand.