I'm put in the mind of someone who uses technomancy. Their primary is cerebromancy and secondary is electromancy, they don't necessarily build machines but they use their primary to bend the "wills" of machines (such as an AI) and their secondary to manipulate the body.
I got a question though, if the Crystal Mirror reveals someone has a "dark" aura such as the one for Necromancy, what's the procedure then? Is that student pulled from the side to be vetted for secret necromancy classes? What if they're really unfortunate and have two dark mana shades? Such as both necromancy and hemomancy?
Well, bending the "will" of anything sounds more like Psychomancy than Cerebromancy, but neither would have an effect on an automaton without a soul. And if you want to move machines' bodies, Geomancy with a focus on metallic substances would fit best. So, to have something akin to a technomancer, I would suggest Geomancy as the Primary Arcane Art and Electromancy as the Secondary Arcane Art.
The Crystal Mirror was only designed to show the eight primary Mana shades because those are the only ones that someone is born with affinities towards. The various shades of Black Mana are beyond the grasps of anyone who has not specifically trained to harness them. Let's envision the Aetherscape as having an Inner Core where the eight shades reside, and then an Outer Void where Black Mana is drawn from.
In Ancient times, people only knew about the eight Mana shades, but it was the efforts of overzealous Cerebromancers who sent mental projections of themselves deeper into the Outer Void that led to the discovery of Black Mana.
Everyone, please feel free to ask more questions and help me further develop this RP world in the process. š I'll work on some more in-universe official terms for stuff as well.