dusk, dark confused, stupid
昬 centers on the fading of light and clarity: dusk, darkness, and the mental confusion or dullness that comes with it. The 'stupid' sense is an extension of that dim, unclear state.
昬 is a variant form of 昏, combining 日 (sun) with 民 (people) instead of 氏. The original 昏 uses 氏 over 日 to suggest the sun setting low, representing dusk. The variant with 民 likely arose from visual confusion or simplification.
The sun 日 is setting below the people 民, bringing dusk and darkness. When light fades, things become confused and unclear.
For コン, imagine a con artist using the dim light of dusk to trick confused people: con -> コン, and the darkness hides the deception.