I'm not sure I totally understand your question--when you ask why their skin looks "obscured" in those pictures, do you just mean "why does this European king look dark-skinned?"

I don't know about Boris I specifically, but the simple fact is that not everyone in Europe is particularly pale-skinned, and especially in the Balkans/the Byzantine Empire it isn't unheard of for people to be dark-skinned. One example of a dark-skinned ruler off the top of my head is Manuel Komnenos. It was unusual enough that he was mocked for it:

This evil they compounded by inflicting an even more monstrous one: they stole an imperial ship, adorned the imperial cabins with curtains interwoven with gold thread and with rugs of purple, and placed on board an accursed manikin, a certain black-skinned Ethiopian. They acclaimed him emperor of the Romans, and led him about in procession with a splendid crown on his head, ridiculing the sacred imperial ceremonies and mocking Emperor Manuel as not having yellow hair, the color of summer, by instead being blackish in complexion like the bride of the song who says, "I am black and beautiful, because the sun has looked askance at me".

--from the Annals of Niketas Choniates, describing Venetians mocking the Emperor after capturing an Imperial ship.

So basically I think the answer to your question is that there are depictions of European rulers with dark skin because.... there were European rulers with dark skin. Southern Europeans and Mediterranean people in particular have a diverse range of appearances, so I don't see anything so far-fetched about that.

Follow up: what's in his left hand?