I think it's better not to treat the vague obscurement rules as defining "the physics of light" in D&D--it can produce problems like transparent creatures and/or walls. I'd much rather treat the obscurement rules as an abstraction of the real world. So at my table, if a creature in natural darkness is visible as a silhouette, I infer that they, exceptionally, must not be heavily obscured for rules purposes. The gameplay implication of that choice is that characters should take the location of light sources into account when trying to lurk in the darkness, and I'm totally fine with that additional bit of realism.