The black budget is a secret budget for covert operation. How can the U.S. government ensure that the money is spent appropriately and people don't spend it for personal reasons or other reasons not related to national security or protects the national interests of the country.
Such a budget will, inevitably, fall under an executive department, headed by either a Cabinet Secretary, or the President himself. The budget for that department is scrutinised by Congressional Committees.
Both the relevant Cabinet Secretary, and the senior members of the relevant Congressional Committees, are allowed to know about relevant classified information (including details of such operations), and to scrutinise in a way they see fit. They just have to keep this secret.
It can't and therefore doesn't.
Occasionally this comes to light. A large amount of money related to the rebuilding of Iraq was simply stolen.