I rule if there is a light source and something to block that source there is a shadow i.e the sun shining behind a house you get a shadow in the pitch black dungeon you don't.
You could say that but I wouldn't rule it that way.Isn't being in a pitch black dungeon simply being in the shadow of the earth?
I rule that the area must be dim-light or darkness...as in the 5' square you are bampfing in or out of must be lightly obscured do to lighting conditions.