That assumes that civilizations derive power by encasing their host star in a Dyson sphere, which is not a foregone conclusion; that prediction was based on our current understanding if technology. Think of Huygens who imagined that Venusians sailed the seas on ships held together with hempen rope.
An "amorphous blackbody" source, from anything reasonably distant, just looks like a smaller star. There are a bunch of if's and unknowns involved, but it could be maybe possible to detect a Dyson sphere. This[1] article talks about it. The upshot is that it is nowhere near blindingly obvious, and we don't currently have the ability to differentiate between a natural or artificial infrared source.
