“Blue money” describes funds that are legitimately earned, but are not reported to all the appropriate governments for tax purposes. The term “blue money” has been cited in print since at least 2006 and was popularized in Argentina.
 
 
Expat Argentina
THURSDAY, JANUARY 19, 2006
Bringing Money Into Argentina
(...)
Even though the financial firm you are dealing with may be a well respected legal entity here in Argentina, the operation they just carried out for you is extra-legal. The law of the land is that all transfers must go through the Central Bank. Someone told me the term for this is “blue money”—not quite black money from drugs or something else illegal, but not exactly 100% legal either.
 
Google Books
Global Securities Markets:
Navigating the World’s Exchanges and OTC Markets
By George W. Arnett
Hoboken, NJ: John Wiley & Sons, Inc.
2011
Pg. 139:
White Money, Blue Money, and Black Money
(...)
“Blue money” is those funds that are legitimately earned, but are not reported to all the appropriate governments for tax purposes. These funds move through unlicensed and unregulated intermediary accounts set up by or for a local casa de cambio, casa de bolsa, travel agency, or other financial institution or entity. These intermediary accounts are typically shell companies set up outside of Argentina that are not subject to the review of a regulator.
   
The Wall Street Journal
April 12, 2013, 7:27 p.m. ET
WEEK IN WORDS: ERIN MCKEAN
white money
But crackdowns by tax authorities in the U.S., U.K. and Europe in recent years have forced the banks here to hone so-called white-money strategies, which focus on managing only offshore assets declared for tax purposes.

“Credit Suisse Threatens to Cut Off German Clients,” Europe Markets, WSJ.com, April 7

White money is money that is legitimately earned, and fully reported for tax purposes; blue money is legitimately earned, but not reported to every government that could legitimately tax it; and black money is neither legitimately earned nor reported.