Man, people keep going on about how the restoration of the Sunwell was a mistake, and for what reason? The race evolved. The Sunwell's been restored in real time now than it was destroyed, and their phase of fel/desperation was actually brief in the overall history of the race now.
Did they start there? Yes. Are they that now? Absolutely not. The Blood Elves have, still are, and will always be the quintessential paladin race of the Horde.
Holy theme customization makes the most sense for them going forwards, and if people aren't a fan of that, they can either not use those options, or they have another option with the more chaotic/gothic void elves, if they can stomach going Alliance. (Which is somehow the end of the world to some people to get what they want.)
You can play a High Elf on either faction, but it's extremely clear with the story that those elves have split off with a major themeatic. Alliance has Void/Gothic traits, and the Horde has Holy/"Royal" traits to them. They mirror each other, and there's been hints of a Light vs Void plot coming.
Also, sorry, red eyes absolutely suit Void Elves way more than Blood Elves. (Red is a void colour in both WoW, and across media in general.)
And San'layn are not Horde. They failed to join the Horde, betrayed them after Talanji stated she already didn't trust them (do Alliance quests), and the Void Elves already cover a lot of the requests san'layn players have had like hairstyles, and the eye colours
except red. If those options are going to anybody, it's likely them because they have almost the entire kit covered anyways, and you can make one on retail now if you can live without red eyes - especially as a DK.
When it comes down to ease of implementation, if it's as simple as one eye colour, and a bonus of a single ear shape and maybe tints to existing skins, it's viable.
Also, Dar'khan was scourge, had nothing to do with the void, yet played a major part in Umbric's backstory? Either they were going to be san'layn and it got changed in development, or there's a huge opening right there for a subrace recruitment.
They're about as "blood elf" as a void elf is, but the void elves actually can use red - a void colour - to make them a subrace with very little work compared to blood elves, who'd need to have void elf assets to make it work for them.