To make things visually more interesting and the sunscreen concept more realistic.
Aura of darkness

The big question here is how should that be done. The easiest way is to spawn particles. All other ways seem complicated.

On second thought, this would probably also require a name change if its not a sunscreen applied to your skin but rather a dark aura. But I guess it would be cooler.

Yeah, no idea how to implement it. Maybe just dense dark particle, but maybe there is some other fancy way.