During the pandemic, some people sold masks at high prices on the black market.

black market 是由形容词 black（黑色的）和名词 market（市场）组成的复合词。black 在这里比喻"非法的、秘密的"，market 指"交易场所"。black market 指"非法交易的市场"，即"黑市"。