The night elves and blood elves are only seperated by about 10,000 years. Prior to the sundering, Highborne (the ancestors of the blood elves) were a caste of nobility that held themselves aloof from from low born elves but they were both part a single society, and were not two seperate species living side by side; they were like nobles and commoners in a feudal culture.
A seperation of only 10,000 years is less than the period that various human races in the real world have been separated from each other, but all human races that currently still exist area single species ho-mo sapiens sapiens.
Genetic and fossil evidence is interpreted to show that archaic ho-mo sapiens evolved to anatomically modern humans solely in Africa, between 200,000 and 150,000 years ago, that members of one branch of ho-mo sapiens left Africa by between 125,000 and 60,000 years ago. The date of the earliest successful "out of Africa" migration (earliest migrants with living descendents) has generally been placed at 60,000 years ago as suggested by genetics, although attempts at migration out of the continent may have taken place as early as 125,000 years ago according to Arabian archaeology finds of tools in the region. The earliest divergent brances of humans are the khoisan of Africa, the andamanese in the indian ocean and the Australian aboirigines, all about 70~50k years diverged from the other out-of-africa humans (European/Asian/American races).
Africans, Eurasians, Australians, Austronesians are all races of Humans, but they are a single species of human. Blood elves and night elves are both races of elves; but blood elves and night elves are a single species. The same is true forest trolls and ice trolls and desert trolls, etc.