PS - I just re-read your post. As far as I know, layer masks can not be transparent. They are either black, white, or some shad of gray in between those two extremes, but they do not support transparency. So, you will never see the usual checkerboard to indicate transparency if you are working on, AND viewing the mask.
In contrast, if you were painting on the layer mask, but viewing the actual contents of the layer (ie, NOT viewing the layer mask), areas where the mask has been painted black will be represented as a checkboard, indicating transparency of the contents of that layer in that area. This, of course, assumes no other layers are turned on.
HTH,
Tom M