I think the elves of Quel’Thalas are half night elf and half human.
Humans came to the northern part of the Eastern Kingdoms 15,000 years before the opening of the Dark Portal. They settled around the area of Keeper Tyr’s tomb, Tirisfal.
5,000 years later the sundering happened, and 2,800 years after the sundering, the High Born Night Elves were banished across the sea, and settled in the area of Tirisfal. It was 500 years after they arrived in the Eastern Kingdoms that these elves sequestered themselves in Quel’Thalas.
It’s during this time that the high elves got shorter, lost their immortality, and their skin tones changed to match human skin tones. I think it’s inevitable that in that amount of time, there was comingling between elves and humans.
Meanwhile, the Night Elves that remained in Kalimdor raised a veil of mist to shroud Kalimdor from outsiders. This prevented the High Born from returning and is probably what truly cut them of from the source of their immortality.
High Born in the eastern kingdoms would have no way of knowing that a mystical barrier had been placed between the world tree and themselves. They would start suffering from magic withdrawn with no understand what was happening or why. Eventually they would also notice the first signs of ageing.
Humans are mortal of course. It’s very easy to imagine that some High Born would blame these problems on the humans that they had brought in to their mix.
In a bid to restore their longevity, they isolated themselves to a peninsula, and avoided as much contact with humans as possible. Over the course of the next 6,000+ years the human blood in the elves of Quel’Thalas would end up distributed fairly evenly, resulting in the High Elves, Blood Elves, and Void Elves we see today.