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It means/implies under the cover of darkness, or within a "black" box - un-seeable...A market place for anything illegal.

Going full on, we could remove the word "black" from our vocabulary. So hex #000000 would now be "abuefho" (some random made-up word) until people conscript that for their own designs and use it pejoratively: "you 'abuefho' jerk!"...

and then round and round we go...

Because the words "Black Market" literally mean and describe the place which the GP was talking about every where in the world. This is not only an American or English thing, many of other languages use the same expression (black + market) to mean the same thing.

https://en.wikipedia.org/wiki/Black_market

I am amused you don't know what it means but you made your mind it's wrong :)