Safer alternative to “opaque”?

As a child I was taught that opaque means doesn't let any light through at all, as opposed to translucent (lets some light through, but diffused/frosted) or transparent (completely clear, lets you see clear images).

However, it seems a lot of people use opaque as synonymous with translucent. Indeed if you ask Google for its definition of opaque, and looks at its definition, usage example (“opaque with steam”), and synonyms, it seems to support the translucent-synonymous usage. I don't know what Google's data source is for their definitions, but it's clearly descriptive of real usage (as most dictionaries are), not prescriptive of how a word “ought” to be used.

I've certainly heard people (mis)use opaque as synonymous with translucent in everyday speech.

So now I'm looking for a single word to use in place of opaque to firmly communicate that the thing being described is completely impenetrable to light.

In writing we sometimes have to be redundant. In case you are misunderstood, I suggest you use "totally opaque" or "one-hundred percent opaque".

I think that with non-transparent you are on the safe side:

• Not able to be seen through; opaque: a work rendered in non-transparent acrylic. (ODO)

• also non-translucent.

Consider lightproof. Sounds foolproof.

Impenetrable by light [TFD]

That said, you can make something lightproof by using an opaque material also. Another similar term is light-tight.