World Update X patch has reinstalled the default taxiway textures
The updates included with MSFS as part of the World Update X have reinstalled the default taxiway sign textures.
If you want to use the Black Square one's, you will need to overwrite the originals (again).
This is only if you want to see the symbols on the signs displayed correctly and the back of the taxiway sign boxes in a black colour.
I have not yet installed World Update X but I have installed the mandatory patches and these overwrite the Black Square textures..