Because they have night elves voices. They don’t even have their own sound bites.
You didn’t pick the best example. By looking at them? Not really. Unless I actually pay attention and look at the eyes. And even then that’s so minor since there are Blood elf NPCs with blue eyes.
Blood elves are the ones still carrying on the legacy of Quel’thalas. They are the high elves that honor those who died.