Got a window filled with black when attempting to start freeCAD's portable app (v. 0.21.1).
Symptoms are described here:
As discussed in the FreeCAD forum, renaming FreeCAD's bin/opengl32sw.dll to opengl32.dll fixed the issue. The Path to the file is something like ...\PortableApps\FreeCADPortable\App\FreeCAD\bin
Hope that helps, thanks!