Have you looked at the back of a doubleshot keycap? What you'll see is a grid pattern in the colour of the legend, similar to the pic below. If the 'blue' bit was transparent you would see the grid and the legend would be unreadable against it, if the legend was transparent most of the back will be covered by the 'blue' so not much light would get through.
And just in case you were going to suggest it GMK keycaps are made of ABS so no, dye sublimation is not an option.
This is why no-one's done it.