It is obtained through
- Battles and Level Taxes on most of Lava Plains, plus the final and bonus scenarios of The Tundra
- Supply Station
- Prison (at random when you make a capture)
- Daily Check (once an month)
- One of the possible rewards you can get from the Daily Login Reward at the Reward Center
You can also buy it in Marketplace.
The max stack of Black Gold is 9999.