black box
n.— «You can add a fourth box to the amber, blue and green boxes the World Trade Organization uses to try to keep straight the “aggregate measures of support” member countries provide to agriculture. The fourth is a “black box,” the term U.S. trade officials have adopted to describe the loopholes some countries are trying to carve out in the ongoing debate over increased market access, the issue that has ground the Doha Development Round to a halt.» —“WTO countries add another ‘box’ to Doha Round lingo” by Forrest Laws Delta Farm Press July 19, 2006. (source: Double-Tongued Dictionary)