Yes, we’re talking about elves (and dwarves, and hobbits, and even Middle-earth humans) but the issue is that you’re applying human ethnic divisiveness to these fictional races where it need not belong.
We do know how humans came to be so varied, and the idea that ethnic isolation and group homogeny is a natural and historically accurate way for populations to evolve (which must in turn be applied to fantasy races) is simply an ignorance on your part on the subject of human evolution and history.
If you were to travel back in time 10,000 years and traverse the European continent you’d find a large variety of skin tones as the mutation that primarily contributes to European white skin was still making its way across the continent. You’d find some tribes and villages where the mutation had fully diffused and most of the population was light skinned, others with mixed skin tones where the mutation was introduced but not fully expressed in all individuals, and even some villages where the mutation hadn’t yet reached and the peoples would still be dark skinned. If you then stuck around for a few thousand years, you’d also see the continued migrations of people from Africa, the Fertile Crescent, and the Near East into Europe, bringing with them things like the domesticated pig, cattle, and horse that helped spur on the Neolithic agricultural revolution. Multiethnic populations existed through ancient Egypt as well as the Greek and Roman civilizations of antiquity. Trade routes that connected Africa and Eurasia existed 1,000 years before the Middle Ages.
You need only look at isolated jungle tribes today to see what might have become of the human race if not for the large amount of migrating, mixing, and mingling that occurred from the Neolithic age and beyond. As civilizations evolved people certainly found more and more ways to divide and isolate each other (wealth, religion, culture, nationalism, etc), but race as a means of dividing people by physical traits only popped up as a concept about 400 years ago.
The idea that populations should be segregated based on skin color is a fairly modern thing. The reason I bring all of this up is because you (and a number of other posters) seem to think that this mentality should be applied to any fictional race that can exhibit human eye, hair, and skin coloration. The idea that different skin colors necessitate cultural division and cannot simply coexist with each other is based purely on racialist conditioning and an overly simplistic view of human history.
You ask how an elf can “randomly be dark skinned”? European white skin was a spontaneous mutation which then took several thousand years with hundreds of generations of traveling and intermixing to become a predominate trait across a continent. The same goes for all variations in hair and eye color (mutations that spread through population mixing). You’re also assuming Arondir is unique in his skin color, but until we’ve met every elf in Middle-earth there’s no reason to make that assumption. It might be a rare trait, but without all the racialist baggage of our modern world there’s no reason why he or any other elf with a similar skin tone would be treated differently. They’re simply elves and that’s it, the way we’re all simply humans regardless of skin color. If anything, the idea that populations should be segregated and isolated based on skin color is a relatively modern idea.