Alright, then lets talk about the Lore.
At the end of the Sunwell the Naruu M’uru had it’s heart used to reignite the sunwell along with it’s embodiment Anveena.
With that reignition the power from the Sunwell became Arcane-infused-Light, combining the energy of the original Sunwell with the light from the falline M’uru. This is where current Blood Knights draw their power from.
Blood Elves no longer have access to the ‘pure’ Arcane energy from the original Sunwell, it’s been polluted by Light. So you have the choice of Green and Yellow eyes to show the ‘old’ and ‘new’ sources of Power.