There
are physiological differences, but similar to the differences between green and brown orcs, they're largely cosmetic.
There was a time, back when the RPG sourcebooks were still considered a viable source for new lore, when it
was indicated that the blood elves as they currently appear are essentially the first stage of their evolution into a distinctly seperate species from high elves, a result of tapping into fel energy to feed their addiction. Their generally ruddier complexion and darker or warmer hair colors were sited as examples at the time, along with their shining green eyes, and certainly this was reflected in-game, especially with the old high elf and blood elf models in
Classic.
Of course, even setting aside retconning, it's most likely that even if the blood elves were once on the path to becoming a new species, the cleansing of the Sunwell likely halted that process, if not reversed it.
That being said, if Blizzard were willing to introduce the high elves as an Alliance Allied Race, I think it would have been just as easily justified as Lightforged draenei, Highmountain tauren, and Dark Iron dwarves.
- Since high elves and blood elves differ culturally, they can easily be given different customization options, including different hairstyles and jewlery. They could expand also customization for both races by adding tattoos. Not only would this help distinguish them visually, it would be a chance to give them both more personality; perhaps the blood elves could tend toward runic brands and battle scars, while the high elves would have more elegant woads.
- In terms of physiology, the existing differences could be reinforced and exaggerated, with high elves tending towards pale skin and fair hair. Given that they've been disconnected from the Sunwell, it could also be argued that they may not appear as physically robust as blood elves, being more lean and lithe.
- In terms of silhouette -- nevermind that the other Allied Races don't have very distinct silhouettes without their Heritage Armor -- it's simple enough to create a new silhouette by altering their builds, hairstyles, and posture. I mean, consider for a moment how distinct the silhouettes of the 26 characters in Overwatch are, and then realize that 23 of them all belong to just a single race: human.
Of course, this is just me being a bit of a devil's advocate. I think with their decision to add the void elves it's obvious that they wanted to do something a bit more overt. But I think if everyone is honest with themselves they'll agree that the distinction between high elves and blood elves is ultimately not that different from the distinction between night elves and Nightborne or mountain dwarves and dark dwarves.