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.
Data visualization of U.S. intelligence black budget (2013)
The United States Department of Defense has a black budget it uses to fund black projects—expenditures it does not want to disclose publicly. The annual cost of the United States Department of Defense black budget was estimated at $30 billion in 2008 but was increased to an estimated $50 billion in 2009.
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.