[notagoodidea]

That's interesting that in German, Dutch and French we will talk about "black work" (schwarzarbeit (DE), travail au noir (FR), zwart werk (NL)) meaning undeclared/unreported work.

I see multiple probable origins on the net : - Coming for the german "schwarzarbeit" during end of 1st, in-between or during the occupation during 2nd WW and translated in French/Dutch from there. - Patron making employees works during the evening/night during the Middle albeit work was restricted only during day time.

I did not really find authoritative sources for the origin of the expression so take that whole bag of salt.

[consp]

AFAIK you are correct. Before the first world war taxation was different in most of those countries and work was not taxed directly at all otherwise only in war years (i.e. the first world war) so the meaning is new. There was no need for a word for something which did not exist.