if you read the notes, the "shadow economy" is basically goods paid for in case and therefore not recorded with GDP, in essence it is not 100% black market illegal goods it is just items paid for in cash...

for example most of craigslist transactions here in the US are paid in cash and in person, never get reported or recorded so that all goes into the Shadow Economy.

US shows the "lowest" numbers, does that mean we have the lowest black market, NO, it just means most of our sales and transaction are recorded more so than other countries.

BUT IMO, the gov and IRS have no clue as to what is really going on in the background, no one can, drug deals, craigslist, prostituion, etc etc, no one can put a real dollar amount only guessing.

BUT this is a great article none the less.