So, to bring things back around, maybe this is the right answer (or, at least a palatable one): that the pre-imprisonment shadow people (and the halflings who created the Pristine Tower) drew on the power of light and darkness somehow to tap the sun and the black, and between that and teaching from Rajaat while imprisoned, they learned to harness that from the otherside to both become the shadow people we now know and to do all the supernatural stuff they seem to need to do.
In all fairness, the shadow people aren’t really halflings anymore, so I suspect that either Rajaat would quietly eliminate them if he even succeeded in his plans, just to keep them from “messing up” his re-perfected Athas.