There is often more than one way to skin a cat.
In this case, create a Label in the same width/height as the card image. Set the color to Black and the Alpha to something between 0 and 255 -- the higher the number, the darker the label. I tried a value of 100 and it looked OK (see attached image), but you might want it darker still - experiment with this value.
Probably the easiest way to implement this approach is to have a dark label for each of the card positions and manipulate the Label's visibility as required.