Talking about shadow as a power source doesn't make much sense in AD&D/3e
Whaaaaaaaaaaaaaaaaaaaat?
Okay, let's talk about what Shadow was in previous editions.
Shadow was not 'That thing that shows the absence of light caused by occlusion from an opaque substance.'
Shadow is not 'The darkness'.
Shadow is not 'The opposite of light and all it means.'
Shadow refered to a substance that illusions could create... not -quite- real, but real enough that it could cause actual harm. 'Quasireal' was the term for it. A Shadow Fireball, if not saved against, would do exactly the same thing a normal fireball would... it would set things on fire. If saved against, it would still do damage. Not as much as a fireball would, but it would do some.
Shadow was the inherent substance that lies and deception were weaved from, given physical form. Nethermancy is what 4e calls it, but originally, it was simply called 'Illusion/Phantasm'
Why were so few spells tied to it? Well, the ones that were had descriptions like:
'Copy the effect of any Conjuration spell of x level or lower'
Shadow was one of the
first power sources the game had ever had. In 4th edition, they made the source of that quasireality into a physical plane, a large one, replacing the ethereal, and included in it necromantic energies.
How is that NOT a power source?
The Shadow Weave isn't really to my tastes, either.
Then dont. There's precident of shadow magic in Forgotten Realms well before Shar did her thing. Again, Shadow magic isn't some new thing, it's been in the game since 1st edition. It's older than the realms themselves.