In the black economy no tax is paid, that is why Greece is bankrupt to the tune of billions.
Take this example.
Bob the builder does a job off the books for £100.
He spends the £100 on a prossie.
She gives the £100 to her drug supplier.
The drug supplier buys stolen goods for £100.
So we have a total of £400 spent and not a single penny paid in tax.