The story
From black + truffle, from being a truffle with a darkish appearance, darker than white truffles.
Etymology from Wiktionary, CC BY-SA 4.0.
black truffle
Today it means: A species of truffle, Tuber melanosporum, darker than the white truffle, most commonly from France but also found in other parts of Europe, that is prized as a culinary ingredient. Full definition →