black-market price
Refers to the price of goods or currency traded illegally, outside official channels. Often used in economic or historical contexts.
After the war, black-market prices soared due to shortages.
Buying dollars at black-market rates is risky.
公定相場 is the official exchange rate or price, the opposite of 闇相場.
Compound of 闇 (yami, 'darkness; illegal') and 相場 (sōba, 'market price'). The term likely emerged during or after WWII when official price controls led to widespread black-market trading.