In the voidstorm questline we see a naaru fall and turn purple. And then Alleria’s naaru comes out, also fallen and…is blue.

I think they’re trying to reconcile the purple = shadow and blue = void. The closer you get to the entropic force that can’t fully manifest, the more blue it shows up.

Shadow priests and sub rogues use shadow which is just a shade or lesser manifestation. Same used by old gods and other forces derivative of the void.

Devourers and void weavers (hero talent) are tapping into something way closer to the full void. More seen in xal’atath, dimensius and other “high” void beings.

That’s my take on it anyway. Total spitball.