I think that the problem with shadow-biology (which seems to me intuitively VERY possible) is not that we wouldn’t be able to detect its signature, but that the range of things that might signal a shadow-biosphere is very large.

(On your point about “camouflage vs. efficiency,” it’s interesting to ponder the polar bear, whose hollow fur appears white, both camouflaging and insulating it. Although I doubt that any Arctic prey can see in infrared, the insulating benefit of the fur coincidentally camouflages the bear in IR, hiding that “signal” from scientists in some “All living objects emit large amounts of heat” shadow biosphere.)

That said, I don’t agree that evolution doesn’t reward compression. Examples immediately come to mind – sunflower seed packing, micro-capillary water transport in plants, maximal environmental quantum efficiency of photon absorption by chlorophyll , slime mold mimicking of Tokyo rail network efficiency .

Would such pressure toward efficiency lead naturally to camouflage as amazing as the blackbody spectrum? I am having trouble thinking of why not. Visual appearance is communicated though the electromagnetic spectrum. This of course applies not just to a “powered spectrum” but also to the more common reflected one that most animals utilize. So what is the relationship between efficiency and camouflage? Forgetting spectra for a moment…morphological maximum information packing, for example holographic kinoforms , appears like white noise without the knowledge of how to decompress it. Some life is just so obvious that we miss it!