While his flesh itself may be black, Mercury's bodily fluids (like blood!) emit UV-A light. This effect isn't obvious (unless you cut the poor 'kin!) but one can observe the induced fluorescence it causes within him easily. Just involves taking a little dive~ Art by Schizy Nylla is Nylla!

Ohhh thats a neat effect, and its always good to see shadekin around!

That looks like a very cozy belly to be in!