One take I have seen on shadow powers is that they don't have any power of their own. Instead, they lead others to fuel their spells for them. This necessitates entwining the mage and their victim, because the victim is the one actually casting the spell -- they just don't know it.
For source material, one can look at the work of great pickpockets, such as Apollo Robins. He does almost nothing at all; all of the work is done by the victim's own mind.
For a slightly more scientific approach, one could look at the curious case of metastability. You can get all sorts of counter-intuitive results there. For example, with metastability, it is completely legal to have a temperature below absolute zero (the rule that nothing can go below absolute zero is for stable systems, and does not apply to metastable systems). Usually heat goes from the hot object towards the cold object. However, with these systems, heat flows from the ultra-cold object into the hot object. This makes it appear that the object with negative temperatures is actually hotter than the hot object it is interacting with!
It would not be a large leap to explore metastable systems where it appears that no information is being transmitted with light (which would appear to be an absolute shadow), but in fact great energy is emitted from that shadow if you try to interact with it.