Yep, you got it: it's the manipulation of the raw shadowstuff of the Shadowfell.From what I understand, although you do use shadow magic, the power comes from the netherworld (or the world below, the inferior world), and Nether actually means 'from below, inferior'. The preffix 'infero-' exists on the dictionary as something relating to inferior or from below so 'inferomancia' is actually a correct translation with the same meaning: infero+mancia, infero- from latin of inferior, -mancia from greek divination.
From what I could see, every spell with the Nethermancy keyword also has the Shadow keyword, there I could call it Umbromancia (umbro sound better here than the alternatives). Also, after that I like the idea of using 'Umbroso' for the Shade race, here is the etimology of this word: lat. umbrósus,a,um 'assombrado, obumbrado, coberto de sombras'. Sounds perfect.
So, based on all that, what was the original desire of Nethermancy? Power for the inferior world, or power from shadows (and therefore the Netherworld in question is Shadowfell)? Do you think D&D could come up with a Shademancy or Shadowmancy?
PS: How would you translated Shade Spell of Nolzur? I used Magia Sombra de Nolzur, but sombra as adjective doesn't sound very nice. When I think about it, I think Shade as projection of what the real spell is.
EDIT: I think I got my answer: Nethermancy lets you command shadow magic in its purest and possibly even deadliest form, shaping it to whatever ends you can imagine.. Taken from: Dungeons & Dragons Roleplaying Game Official Home Page - Article (Necromancy & Nethermancy)
I don't like when I have to use too many connectives, sombra DE magia DE nolzur. Besides, in this particular case, the Spell in the title refers to the new effect. You don't create a shade of other spell, you create a shade spell of another spell (the effect text even mentions it like that). 'Sombra' as an adjective has the correct meaning (an specter, something that resembles another thing, etc), I just don't like the word 'sombra' there as it seems to imply Shadow Magic, which is not true. Compared with "Magia Fantasma de Nolzur" (the same thing, with a more powerful spell) sound nicer in my opinion.Re: Shade Spell of Nolzur: you're right. "Shade" in this context means "a much diminished form" or "a pale imitation of the real thing". In this case, it would be "Sombra de Magia de Nolzur".

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.