“Black gold” is a nickname for petroleum.

It can also be used—less commonly—as a nickname for:

• Compost: “A household produces about a tonne of organic waste annually, she explains; wigglers eat half their weight daily, and in one year, a half kilo of them, numbering 800 to 1,000, and their offspring, can convert that tonne of material into ‘black gold.’”—Ellen Moorhouse, “Learning to love the magic of worms; Red wigglers can turn your garbage into ‘black gold’ so meet vermicomposting advocate Cathy Nesbitt,” The Toronto Star ^[1], September 19, 2009
• Bluefin Tuna: “Fishermen here call it ‘black gold,’ referring to the dark red flesh of the Pacific bluefin tuna that is so prized in this sashimi-loving nation that just one of these sleek fish, which can weigh a half-ton, can earn tens of thousands of dollars.”—Martin Fackler, “Tuna Town In Japan Sees Falloff Of Its Fish,” The New York Times ^[2], September 20, 2009
• Black Peppercorns: “If those vital downpours have not washed away what passes for the road, a few days travel into Kerala’s rolling Western Ghats, where waterfalls roar and herds of wild elephants loom from soft mist, brings you to the ancestral home of Piper nigrum.  High up in the middle of nowhere, Iddicki produces the finest pepper in the world, its peppercorns always dark and heavy, bursting with flavour.  Its vines wind their way around almost every tree in sight, climbing ten metres or more into the sky. After such a journey you might expect Iddicki to be a sleepy backwater.  In its own idyllic way, though, it is a boomtown worthy of the Wild West.  Fancy jeeps clog the narrow streets; shops overflow with the latest necessities of rural life, like washing machines and stereos.  Giant satellite dishes shove their expensive snouts at the heavens from every other house.  One of the world’s largest stashes of gold is in rural India, and to judge by its glittering jewellery shops this town has considerably more than its fair share.  ‘Black gold,’ explains one pepper farmer with a broad grin, is fetching top prices on the world market.”—No author, “The Spice Trade:  A taste of adventure; The history of spices is the history of trade,” The Economist ^[3], December 19, 1998, p. 51