Once you've researched Blackchain (or if you already have blackcoins), blackcoins can be bought with relics. Blackcoin speculation is all or nothing: clicking the "buy bcoin" button will use all your relics; similarly, clicking the "sell b-coin" button will sell all your blackcoins. Blackchain is unlocked by Quantum Cryptography
The price of blackcoins starts at 1000 and will fluctuate slightly daily. There is a 40% chance that the price will increase, a 30% chance that the price will decrease, and a 30% chance it'll stay the same.
Each game day, if the price does not stay the same, then the game generates a uniform random variable in [0, 0.0025%), and increases / decreases the price by that percentage.
Therefore, each game day, the change in ln(blackcoin_price) has mean +1.249 927e-6 and variance 1.442 706e-10.
If the price ever exceeds 1100 the market will implode and the price will drop drastically, but will continue as normal from there.
After a market implosion, the price will be in the range [770, 880.022).
If you always sold blackcoins immediately before the market implosion, then bought immediately afterwards, on average you would multiply your relics by e (≈ 2.718 281 828) approximately every 800 000 game-days; equivalently, you would double your relics approximately every 800 000 * ln(2) = 554 500 game days or 12 days 20 hours in real life.
The price of blackcoins is kept across resets.
With the Invisible Black Hand upgrade, there's a new "Annoy the Invisible Black Hand" button that immediately causes a market crash. It costs more the lower the B-coin price is. This is useful if B-coin price is close to 1100 and you're about to leave for a while. Just sell your B-coin, Annoy the Invisible Black Hand, and rebuy. Then the market won't crash in your sleep.