Where did I say Blood or High Elves were unique to the Alliance ? Why do you always insinuate people stated things they didn't.
You know lore well enough to know they Sunwell was reinstated as a hole/arcane well at the end of TBC which means it was already planned before TBC was released. From that point onwards the Blood Elves stopped abusing the Light.
That's nice head canon but also total bullshit.
Metzen himself said that giving Blood Elves Paladins was break in the lore :
"well this is certainly a break in the lore, how the hell are we going to rationalize this one" --
source around 3:35
So there was absolutely no lore reason for the Blood Knight mount to be identical except in color to the Human and Dwarven Paladin mounts at the time. It was just Blizzard making Blood Elf Paladins as close to Human Paladins as possible and reducing Human Paladin uniqueness in the process.
In the end they came up with a story to explain Blood Knights which has a lot parallels to the Order of the Silver Hand.
Back in the capital city of Silvermoon, Magister Astalor Bloodsworn was not content with this idea. After long months of study and experimentation, he and his fellow wizards learned how to manipulate and corrupt the naaru's luminous energies. In the end the wizards devised a process by which the powers of the Light could be transferred to recipients who had not earned such abilities. Instead of feeding upon the naaru's magic, the blood elves would wield the naaru's Light-given powers themselves.
Lady Liadrin, formerly a priestess, had recently renounced her vows, for she felt the Light had abandoned her people. She learned of the wizards' achievement and volunteered to be the first to bend the stolen powers to her will. With her decision a new order was born: the Blood Knights. These renegade paladins are able to harness the sacred powers of the Alliance's noblest heroes. Most members of the Blood Knights were once part of the Royal Guard – proud defenders of high elven society who came to believe that the Light had failed them in their hour of greatest need. They see their appropriation of the captive naaru's powers as well-deserved justice" --source
Of course you don't think skin tone is important in this case since it doesn't suit you. Green, Red or Orange Blood Elves wouldn't have been nearly as popular but wouldn't have weakened faction diversity as much as it did in TBC.
If you were truly as concerned with faction diversity as you claim to be you would see that playable Blood Elves severely weakened faction diversity. Probably much more than High Elves ever could at this point since they've already been shown as NPCs for years.
Blood Elves were deliberately added to the Horde to give them a human-like race with a human-like skin-tone with the sole purpose of weakening the faction diversity. Blizzard could have chosen other Horde races to have Paladins or they could have chosen a different race for the Horde altogether. Blizzard now bringing up faction diversity as a reason against Alliance High Elves and altering the skin color of Void Elves is therefore pure hypocrisy.