|Institutional slush funds used to allow covert operations to be carried out without need to recourse to on-the-books sources of funding.|
Black budgets are accounts of money which are available for expenses of operations which are covert, i.e. off-the-books. The term "slush fund" has similar suggestions of corruption, but sounds more ad hoc and individual, as opposed to "black budget" which suggests an institutionalised practice of deception.
Wikipedia reports that "A black budget is a budget that is allocated for classified and other secret operations of a nation, a corporation, a society of any form, a national department, and so on. A black budget usually covers expenses related to military research and covert operations. The black budget is mostly classified due to security reasons."
In 2017 Rory Cormac discovered that Stewart Menzies was revealed to have personally operated a black budget of over £1 million during his tenure as head of MI6. This was reportedly given to him by "well wishers" of MI6, including one particularly wealthy American. He disclosed the fund's existence to other MI6 staff on his retirement in 1952. This BBC article concluded noted that "today, MI6 says it operates under the law and under careful political and financial controls."
Wikipedia further reports that "A black budget article by the Washington Post, based on information given by Edward Snowden, detailed how the US allocated $52.8 billion in 2012 for the black budget."
While it is important to acknowledge that government agencies do have access to billions of dollars of funds for use on secret, never-to-be disclosed operations, the black budgets which are measurable are in fact just the tip of the iceberg. Deep state, entirely off-the-books operations net vastly greater amounts (consider, for example, Donald Rumsfeld's spectacularly timed admission (on 2001-09-10) that $2.3 Trillion of funds in the Pentagon could not be properly accounted for. Noting further that it was the accounting department of the DIA in the Pentagon which suffered a hit on the next day, destroying staffers and records concerning to the missing funds, one begins to see that the nominal $56 billion is of only marginal importance in the scheme of things.