Black hat is used to describe a hacker (or, if you prefer, cracker), who breaks into a computer system or network with malicious intent. Unlike a white hat hacker, the black hat hacker takes advantage of the break-in, perhaps destroying files or stealing data for some future purpose. The black hat hacker may also make the exploit known to other hackers and/or the public without notifying the victim. This gives others the opportunity to exploit the vulnerability before the organization is able to secure it.
In cryptocurrency, we saw a lot of exploits done by accessing vulnerabilities in smart contracts. This allowed hackers to drain the liquidity pools of the funds.