Setting transparancy (or Opacity below 95%) always make it black, but if you set above 95% it will help a little this is only trick dont know with others. This happen with my curtain too
If the opacity <95% the material will currently not receive any light from artificial light sources, therefore it looks dark during night time. For curtains like that Iād definitely go for a 100% opaque material with a cutout texture (like you already did) for best results.