Nah it’s not the necropolis, though I can see why it could be.
Every zone in the Shadowlands (bar the Maw) has its own special weather that happens instead of rain. For Maldraxxus, it’s a ‘spirit storm’, the zone goes dark for a bit and if you look up you see a whole bunch of spirits dancing around. It’s real pretty.
When Necropolis is above you can see normal shadow on the ground, so im sure darkness it’s not from it. Im very curious too, about this dark phase. Meybe some old God showing appearance? xD
Aside from potential necropolis it could be a weather effect. I have noticed each zone has it’s own special weather.
Bastion - floating white dots almost like snow.
Ardenweald - Mini comets rain down, purple and make the place darker but also light up fluorescent and burn my retina.
Revendreth - Black rain or raining fire, depending where you are.
Maldraxxus seems to have something… but only seen it a couple times and put it down to a theatre of pain boss effects .