I admit, I have rethought it a bit. If Alleria is ‘the first Void Elf’, and new recruits (High Elf Wayfarers and Silvermoon Scholars) can become Void Elves too (thereby increasing their population to a proper Allied Race), then blond makes sense.
In that scenario, the purple-skinned, Void-infused haired ones would just be the original elves transformed by the ethereals and not the new recruits who joined afterwards.
Yes, this is the most likely scenario at the moment until proven otherwise. I have written it before somewhere but I think that the other way around it would have made more sense in the long run:
Playable High Elves as the first AR and they unlock during the Argus scenario the Void Elf abilities. This would not only serve a better connection to the given lore at this point but would also rather have interesting internal conflicts between the Silver Covenant and those new Void Elves. The Void Elves would have been the natural progression of the Alliance High Elves and people could either root for the natural or the voidy ones. Both skin variations would be available without shoving an unnatural story progression into the player’s mind.
It’s also possible that the process that new recruits are going through is like Alleria’s, but takes some shortcuts to accelerate their mastering of the void, and maybe accidents happen, resulting in some of the new recruits having the blue skin, or tentacle hair, or both. This sort of explanation would ensure that the blue skin and tentacle hair remain features of at least some of the future Void Elf generations.
Having some Void Elf NPC’s with the natural skin and hair tones, some mixed with both the original voidy and natural options, and some being full on voidy blue tentacle haired, would show that the Void Elves can come in a variety of appearances, and that the blue skin/tentacle hair isn’t just for the first generation.
Maybe down the road, we’ll find out Void Elves can indeed procreate in the usual fashion. And perhaps their offspring will inherit void skin/hair traits, but also a resistance to the whispers (because I am not sure how a child would be able to survive them otherwise).
Obviously that’s all headcanon. We’ll have to see where Blizzard takes the canon Void Elf story… assuming they bother to do anything with it at all.
When it comes to what these options represent it could honestly be both. Second Gen Void Elves who are Silvermoon Scholars/High Elf Wayfarers. But it could also just be they’re to represent plain old High Elves as well. In the same way that Wildhammers are represented by the playable Bronzebeard Dwarves despite the fact the racials and some other things don’t really match.
Or the fact that several Orc Clans are all rolled in under Mag’hars.
Or that Farakki Trolls & Dark Trolls are now rolled in under the Darkspear Jungle Troll race.
Regardless though it would be cool to see some of the old High Elf npcs around the world get updated to use the “Void” High Elf customization options. Create more of a difference between them and Blood Elf NPCs.
They need an entropic embrace toggle in character creation.
Honestly, I wonder if they could do a “Race Name” toggle. Though… you’d be able to make some purple High Elves that way.
The better option is to give them a new name entirely for the group.
On one hand, there are ways to figure out how to cast Void Magic that don’t involve eating a dark Naaru or being corrupted by a prince. Honestly, she ate a dark narrow, Alleria may very well be able to change people into Void Elves…
The question is… Why would you want to?
No more lore breaking than void elf itself. Maybe they dye their hair or use a glamour. Be creative about it. The void doesn’t seem to be a 1;1 in how it effects people because Alleria herself is still blonde and all her voidy powers.
It’s also known that high elves literally join the Void Elves to study void magic, maybe you’re just not a first gen ‘I was made half void’ void elf.
Surprise FFXIV 
Magic is really the answer 9 out of 10 times to most problems.
A wizard did it might be lazy but it works.
If I were to guess (speculative headcanon), the primary incentive I can see for elves wanting to join the Ren’dorei is rooted in overcoming what is viewed as one of most malefic cosmic forces. While we only see Uldum and The Vale of Eternal Blossoms get invaded by N’zoth in game, it’s possible that other areas of the world were affected. Not to mention the whole Ny’alotha raid.
The forces that dealt with the assaults and Ny’alotha itself, got a good close look at the power and danger the void represents. Seeking mastery over that power, so as not to be overwhelmed by it, or even to turn it to beneficent ends, could be quite the motivator for some to seek out the Ren’dorei and join them, despite the risks that come with it
With that said, the above explanation is a bit shaky because that same incentive would apply to everyone, not just Thalassian elves. I mean, it could be explained that the magic assimilating nature of elves is what makes it possible for them to master the void as they have, but that would mean that Kal’dorei/Shal’dorei could probably also master it in the same way if so inclined.
And of course there’s age old cliche “for power!” as incentive. 
I just think that with how much of a role the void and the old gods have played in the shaping of races on Azeroth, either directly, or through the effects of actions taken by those who wield it, that many would seek to understand and master the void in order to protect themselves from it, if nothing else.
My preferred explanation that I want them to implement is that Umbric and his magisters have been researching the ritual that changed them, Locus-Walker and Alleria being there means they can refine the path she took.
So they’ve refined the training bit to not take a thousand years with what they now know through Alleria and how it affects elves.
They now have much of the ritual that changed Umbrics group into Void Elves.
And there is a large Void Storm on the third island.
I want them to contain that storm with some sort of magical building/edifice. Think Altar of Storms looking but… Void Elfy.
Then using Alleria and Locus-Walkers refined training regime train up new recruits to resist and control the void.
Then bring it all together using a less damaging ritual with the Void Storm as a catalyst to infuse the void into new void elves.
I want Blizzard to show this in game, and have the results of that final step have void elves comes out with either a random set of customizations available to players or just have a few NPCs with mixes, some with blueberry looks like Umbrics, and a few like Alleria untouched.
That way we get lore on how we’re made, lore in general, and everyone’s options get to be canon void elves without any room for argument. Plus newer players wont have to seek out a vague reference in the ether to know how new members are made.
Doesn’t really make sense for the majority of High Elves…
and it’s a good way to end up corrupted and turned into weird void sludge. It’d be like a bunch of Gnomes wanting to become leper gnomes for the power they wield.
Don’t get me wrong, I like Void Elves and I’ve got a few. But there is no reason why any elf would want to replicate it.
I guess, perhaps, Alleria can be some kind of role-model but in the three sisters comic its shown that even she is barely holding it together sometimes.
I think a racial tag/subrace system might be the next big thing when it comes to new races/allied races. Specifically being able to pick that subset within a race including the proper tag and perhaps even racials.
High Elves becoming the “Sub” option for Void Elves. Opening up the Paladin option.
Wildhammers becoming the “sub” for Dwarves. Opening up druids while losing Paladins/Warlocks/Mages.
San’layn/Dark Rangers becoming the “Sub” For Blood Elves while losing paladin option.
Would be nice if we got something like this if it also came with more heritage gear or a mount.
When it comes to Void Elves/High Elves (assuming the subrace thing i suggested never happens) i think this would be a good choice tbh. Something that can encompass BOTH the standard High Elves of the Alliance as well as Void Elves. I’ve suggested “Allerian Elves” before but im sure there could be a better name if need be.
That recruiting poster…
“You didn’t become Blood Elves because you refused to drain mana. But just consider, if you do a crazy experiment with void magic that may turn you into a tentacle monster, then you can hear whispers telling you to kill your family for the rest of your life.”
Void Elves are unfortunately a dead end for rational lore. They’re a small squad of Blood Elves… and if other elves have any sense it’ll stay that way.
Once again, I do like them. It’s just nonsensical anyone would pursue it.
They aren’t recruiting per se…
Or so I’m told. 
These elves are just for whatever reason showing up. Thats just in the lore already. Its where most of the theories on how more Void Elves are made come from.
Most of them seem to be happy that they’re no longer being restricted by their text. So I’d assume these elves just have some interest in the void.
Don’t really need an explanation for the why. People do things for all sorts of reasons. Elves do have a tendency it seems to seek power regardless of group involved.
Its like asking why a human goes to do a thing.
The void power up should have been just a key to press and power up, literally. Would have saved so much trouble about the entrope discussion. But this was bound to happen if you deny the people their High Elves and have to backpaddle the whole situation.
I hope so too. Still cannot believe that they added three (!) human races into the slots.
Agreed.
With the addition of the toggle option, we’ll then be able to have free reign to mix and match, creating a spectrum from a fully void-embraced void elf, to one almost high elven like Alleria.
We can only go forward from here for more customization, together.
Yes. At this point there isn’t a good reason not to do something like what I mention above.
It’ll be beneficial to the game and the lore. Plus it doesn’t hurt high elves or the people who want to play them in any way.
Just because they implement something like that doesn’t mean you’re not playing one of the ones still training for instance.
Absolutely! And I hope the toggle option allows us to add tentacles to hair options that didn’t have them, thus allowing a full range. New things we get won’t just be for the ‘high elf’ or the ‘voided up void elf’, it’ll be for our race as a whole.
Not to mention tattoos and a couple more hairstyles, we’d be set for good.
You mean Worgen?
I suppose it’s true.
Mechagnomes, Lightforged Draenei are also basically High Mountain Tauren. You guys didn’t have to make a new race for those customization options.