What is Black Money Scam?

Legal Definition
The black money scam, sometimes also known as the "black dollar scam" or "wash wash scam", is a scam where con artists attempt to fraudulently obtain money from a victim by persuading him or her that piles of banknote-sized paper in a trunk or a safe are actually currency notes that have been dyed to avoid detection by authorities. The victim is persuaded to pay fees and purchase chemicals to remove the dye, with the promise of a share in the proceeds.

The black money scam is a variation of what is known as advance fee fraud.
-- Wikipedia