Use the adjective opaque either for something that doesn't allow light to pass through (like a heavy curtain) or for something difficult to understand (like bureaucratic gobbledygook).

Opaque is from a Latin word meaning "dark," and that was its original sense in English, but it now means literally "not transparent" or metaphorically "hard to understand; unclear": "Some of his sentences are really opaque." It used to be spelled opake, which made the pronunciation clearer, but we then borrowed the more opaque French spelling. Come to think of it, English spelling is often pretty opaque, which makes it hard for learners of the language to master.

Definitions of opaque

adj not transmitting or reflecting light or radiant energy; impenetrable to sight

opaque windows of the jail”
opaque to X-rays”
not clear to the mind
cloudy, mirky, muddy, murky, turbid
(of liquids) clouded as with sediment
fogged, foggy
obscured by fog
(of glass) having a roughened coating resembling frost
having a frosted look from a powdery coating, as on plants
light-tight, lightproof
not penetrable by light
milklike, milky, whitish
resembling milk in color not clear
partially opaque
impenetrable for the eye
allowing light to pass through
crystal clear, crystalline, limpid, lucid, pellucid, transparent
transmitting light; able to be seen through with clarity
hyaline, hyaloid
resembling glass in transparency or translucency
limpid, liquid
clear and bright
semitransparent, translucent
allowing light to pass through diffusely
not made opaque or cloudy by sediment
(of glass) lacking a frosted coating
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adj not clearly understood or expressed

incomprehensible, uncomprehensible
difficult to understand

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