To say that something is "never indistinguishable"

Is to say that something is "never indistinguishable" poor grammar? We rarely, if ever, see such phraseology in English, but my understanding is that it is grammatically valid, even if it would be regarded as "poor grammar"? After all, one could simply say that something is "always distinguishable", which carries the same meaning and sounds much more linguistically valid in the English language. As to why it is poor grammar, it seems to me that this might fall under the category of a double negative? So it would be valid, albeit, considered poor, grammar?

I would appreciate it if people would please take the time to clarify this.

Far from the poorly structured and carelessly uttered double negatives, double negative adjectives or adverbs like the one you have mentioned are perfectly fine and often used by eloquent speakers and writers of refined taste.

Consider the following:

She was not unattractive.

Would you say the meaning is the same as below?

She was attractive.

I wouldn't! :D

I don't think "never indistinguishable" and "not indistinguishable" are equal either. I perceive "not indistinguishable" to be somewhere between "distinguishable" and "indistinguishable."

Saying that something has been "never indistinguishable" sounds like many have believed the subject to be "indistinguishable" while it's being said now that it's at least not totally indistinguishable.

More on the subject:

A Comprehensive Grammar of the English Language, Section 10.66, p. 791. Quirk, Greenbaum, Leech, and Svartvik. Longman, 1985.

Credits to @cacambo for providing a link to the rhetoric device:

https://en.m.wikipedia.org/wiki/Litotes