Mars is not black. He's originally quite pale, all demons of noble heritage are white since they live underground.
Canonically, Mars tanned his skin because he would rather be mistaken for a lowborn human, than a noble human. It helps him fit in better in the more populous cities, even though his height makes him stand out easily.
Mars is that color, because the game originally was supposed to be a NTR legend ripoff.
But as I was writing the story, I included that subversion.
Also, the BBC thing is totally a western thing that is now influencing some japanese artists due to twitter and fan commissions.
The standard Japanese NTR guy is the ugly bastard or the tanned playboy. Mars mimics the second form obviously.
I honestly only care about Japanese tropes, and mimicking Japanese games/VNs that I enjoyed.
The tanned playboy is tanned because it implies that he spends time outside hitting on girls. This is why in doujins they're often put in beach scenes.