We try to do something similar. You haven't mentioned this but I assume you use a window for your inventory. You can create an image with the dimensions of your inventory window (which must match the dimensions of your scene). Set the colour as black (or any colour you wish) and make it transparent (you can achieve this with an image application, we use GIMP). Then you can set it as background of your window and when you open it its transparency might give you the effect you need.