Upon further inspection the glaze is- as you might guess- thinner at the very edges. This lets a small amount of light bleed through with the backlight turned up. It’s much easier to see with a bright mobile phone torch though!
With the backlight at full brightness it shines through both the white keycaps and the very edges of the black keycaps. It looks almost like a trick of the eye in the case of the latter. These caps are so very, very dark gloss black! image
Squeeeeee! image