The comment in the link I posted suggested it as a rule and suggested
that as the title, so unless they were cribbing it from somewhere else
it's recent and probably restricted in use to the Tor boards.

On 05/03/2021 19:20, Kurt Feltenberger wrote:
> On 3/5/2021 12:29 PM, Cian Witherspoon wrote:
>> For those who don’t want to search through the linked pages, The
>> Little Black Rule is “Traveller always has an example”. Considering
>> the sheer amount of “oh that’s a cool idea” over the years, the list
>> of examples of various sci-fi concepts in Traveller is staggering.
>
> Interesting.  I've been playing the game since 78 or so and never
> heard the term.
>